Halachic requirements to allow men to pray in the presence of women differ somewhat from modesty requirements incumbent upon the woman when in the presence of others. Appropriate dress (according to the lenient opinions) when praying in the presence of others, is as follows:

The halacha is that "tefach b'isha ervah" - a tefach (about 3.5 inches) uncovered on certain parts of a woman's body is ervah - nakedness. (B'rachos (24a) in the name of Rabbi Yitzchak). The arm must be covered at least to the elbow and the leg must be covered at least to the knee.

If a married woman is dressed with uncovered hair where uncovered hair is common among married women (though in violation of halacha), according to the Aruch HaShulchan (Orach Chaim: 75; 7) who rules leniently, he writes that it is permitted to pray in the presence of these women (quoting the Mordechai who quotes the Raviah).

Rabbi Getsel Ellinson (Woman and the Mitzvot: Volume 2) - Pg. 186, note 37, writes that it is probably okay to pray as long as there's less than a tefach uncovered above the elbows and knee. However, it is certainly absolutely forbidden to pray in the presence of a woman whose skirt does not cover past the knees when she is sitting down, because the skirt will not cover ervah.