it is indeed not so "clear cut". here is a halachic discussion from Rabbi Mordechai Willig which sheds light on this

Notwithstanding the immutability of the Torah's principle of modesty and its particular application to women, the precise details are subject to communal standards which often change and/or vary from place to place. This is true regarding some parts of a woman's body which must be covered (Shulchan Aruch Orach Chaim 75:1). Nonetheless, there are other parts which must be covered regardless of communal standards.

The Mishna Brura draws the line at the elbow and the knee (75:2). Some interpret "shok" (Berachos 24a) as the calf (since the thigh is called yerech), and include it in objective erva (see Chazon Ish Orach Chaim 16:8). Yet others imply that since the requirement to cover the arms and legs is das Yehudis (Kesubos 72a), i.e. a custom of Jewish women (Rashi), it may be subject to change (see Kaf Hachayim 75:2, Igros Moshe Even Hoezer 1:69). Sha'ar Hatziyun 75:5 disagrees.

However, a woman's torso is certainly ervah (see Rambam Krias Shma 3:16), and must be covered. Unfortunately, many otherwise observant women follow fashions, such as very low necklines, which expose the flesh inappropriately. Women who wear tight-fitting clothes which explicitly delineate a woman's figure are also in violation, as the Midrash, contrasting Rus and the other women, implies (see Kuntres Dinei Malbush Nashim page 12, 13).

The distinction between variable details and timeless principles is not limited to dress. It applies, in a more complex and nuanced way, to the definition of tznius in the Torah society. For example, public speaking by a woman in front of mixed audiences is commonplace in some circles and unheard of in others. For many parts of Torah society, it depends upon the place, the occasion, and other factors. Similarly, interaction between men and women, another subject of the Midrash about Rus, is also dependent upon local custom (Beis Shmuel 62:11, see Otzar Haposkim there). This includes separate seating, entrances, mechitzos, etc. Here, too, context is clearly critical.

It must be noted that the opposite of tznius is pritzus (Kesubos 3b), a term linked to one who breaks a fence (Koheles 10:8), and different communities legitimately build their fences in different places. As such, a garment, speech, or event can be labeled as pritzus in one place, but be acceptable in another.

from http://www.torahweb.org/torah/2010/parsha/rwil_bamidbar.html [1]: http://www.torahweb.org/torah/2010/parsha/rwil_bamidbar.html

it is indeed not so "clear cut". here is a halachic discussion from Rabbi Mordechai Willig which sheds light on this

Notwithstanding the immutability of the Torah's principle of modesty and its particular application to women, the precise details are subject to communal standards which often change and/or vary from place to place. This is true regarding some parts of a woman's body which must be covered (Shulchan Aruch Orach Chaim 75:1). Nonetheless, there are other parts which must be covered regardless of communal standards.

The Mishna Brura draws the line at the elbow and the knee (75:2). Some interpret "shok" (Berachos 24a) as the calf (since the thigh is called yerech), and include it in objective erva (see Chazon Ish Orach Chaim 16:8). Yet others imply that since the requirement to cover the arms and legs is das Yehudis (Kesubos 72a), i.e. a custom of Jewish women (Rashi), it may be subject to change (see Kaf Hachayim 75:2, Igros Moshe Even Hoezer 1:69). Sha'ar Hatziyun 75:5 disagrees.

However, a woman's torso is certainly ervah (see Rambam Krias Shma 3:16), and must be covered. Unfortunately, many otherwise observant women follow fashions, such as very low necklines, which expose the flesh inappropriately. Women who wear tight-fitting clothes which explicitly delineate a woman's figure are also in violation, as the Midrash, contrasting Rus and the other women, implies (see Kuntres Dinei Malbush Nashim page 12, 13).

The distinction between variable details and timeless principles is not limited to dress. It applies, in a more complex and nuanced way, to the definition of tznius in the Torah society. For example, public speaking by a woman in front of mixed audiences is commonplace in some circles and unheard of in others. For many parts of Torah society, it depends upon the place, the occasion, and other factors. Similarly, interaction between men and women, another subject of the Midrash about Rus, is also dependent upon local custom (Beis Shmuel 62:11, see Otzar Haposkim there). This includes separate seating, entrances, mechitzos, etc. Here, too, context is clearly critical.

It must be noted that the opposite of tznius is pritzus (Kesubos 3b), a term linked to one who breaks a fence (Koheles 10:8), and different communities legitimately build their fences in different places. As such, a garment, speech, or event can be labeled as pritzus in one place, but be acceptable in another.

from http://www.torahweb.org/torah/2010/parsha/rwil_bamidbar.html

halachic discussion from Rabbi Mordechai Willig

Notwithstanding the immutability of the Torah's principle of modesty and its particular application to women, the precise details are subject to communal standards which often change and/or vary from place to place. This is true regarding some parts of a woman's body which must be covered (Shulchan Aruch Orach Chaim 75:1). Nonetheless, there are other parts which must be covered regardless of communal standards.

The Mishna Brura draws the line at the elbow and the knee (75:2). Some interpret "shok" (Berachos 24a) as the calf (since the thigh is called yerech), and include it in objective erva (see Chazon Ish Orach Chaim 16:8). Yet others imply that since the requirement to cover the arms and legs is das Yehudis (Kesubos 72a), i.e. a custom of Jewish women (Rashi), it may be subject to change (see Kaf Hachayim 75:2, Igros Moshe Even Hoezer 1:69). Sha'ar Hatziyun 75:5 disagrees.

However, a woman's torso is certainly ervah (see Rambam Krias Shma 3:16), and must be covered. Unfortunately, many otherwise observant women follow fashions, such as very low necklines, which expose the flesh inappropriately. Women who wear tight-fitting clothes which explicitly delineate a woman's figure are also in violation, as the Midrash, contrasting Rus and the other women, implies (see Kuntres Dinei Malbush Nashim page 12, 13).

The distinction between variable details and timeless principles is not limited to dress. It applies, in a more complex and nuanced way, to the definition of tznius in the Torah society. For example, public speaking by a woman in front of mixed audiences is commonplace in some circles and unheard of in others. For many parts of Torah society, it depends upon the place, the occasion, and other factors. Similarly, interaction between men and women, another subject of the Midrash about Rus, is also dependent upon local custom (Beis Shmuel 62:11, see Otzar Haposkim there). This includes separate seating, entrances, mechitzos, etc. Here, too, context is clearly critical.

It must be noted that the opposite of tznius is pritzus (Kesubos 3b), a term linked to one who breaks a fence (Koheles 10:8), and different communities legitimately build their fences in different places. As such, a garment, speech, or event can be labeled as pritzus in one place, but be acceptable in another.

see there for more http://www.torahweb.org/torah/2010/parsha/rwil_bamidbar.html [1]: http://www.torahweb.org/torah/2010/parsha/rwil_bamidbar.html

it is indeed not so "clear cut". here is a halachic discussion from Rabbi Mordechai Willig which sheds light on this

Notwithstanding the immutability of the Torah's principle of modesty and its particular application to women, the precise details are subject to communal standards which often change and/or vary from place to place. This is true regarding some parts of a woman's body which must be covered (Shulchan Aruch Orach Chaim 75:1). Nonetheless, there are other parts which must be covered regardless of communal standards.

The Mishna Brura draws the line at the elbow and the knee (75:2). Some interpret "shok" (Berachos 24a) as the calf (since the thigh is called yerech), and include it in objective erva (see Chazon Ish Orach Chaim 16:8). Yet others imply that since the requirement to cover the arms and legs is das Yehudis (Kesubos 72a), i.e. a custom of Jewish women (Rashi), it may be subject to change (see Kaf Hachayim 75:2, Igros Moshe Even Hoezer 1:69). Sha'ar Hatziyun 75:5 disagrees.

However, a woman's torso is certainly ervah (see Rambam Krias Shma 3:16), and must be covered. Unfortunately, many otherwise observant women follow fashions, such as very low necklines, which expose the flesh inappropriately. Women who wear tight-fitting clothes which explicitly delineate a woman's figure are also in violation, as the Midrash, contrasting Rus and the other women, implies (see Kuntres Dinei Malbush Nashim page 12, 13).

The distinction between variable details and timeless principles is not limited to dress. It applies, in a more complex and nuanced way, to the definition of tznius in the Torah society. For example, public speaking by a woman in front of mixed audiences is commonplace in some circles and unheard of in others. For many parts of Torah society, it depends upon the place, the occasion, and other factors. Similarly, interaction between men and women, another subject of the Midrash about Rus, is also dependent upon local custom (Beis Shmuel 62:11, see Otzar Haposkim there). This includes separate seating, entrances, mechitzos, etc. Here, too, context is clearly critical.

It must be noted that the opposite of tznius is pritzus (Kesubos 3b), a term linked to one who breaks a fence (Koheles 10:8), and different communities legitimately build their fences in different places. As such, a garment, speech, or event can be labeled as pritzus in one place, but be acceptable in another.

from http://www.torahweb.org/torah/2010/parsha/rwil_bamidbar.html [1]: http://www.torahweb.org/torah/2010/parsha/rwil_bamidbar.html