Why is the Taz's position on tefillin parsha spacing controversial?

Question

When writing tefillin, one should ideally leave a setumah gap between the parshiyot of shema and vehaya im shamoa.

Shulchan Aruch Orach Chaim 32:36 holds that this should be done following the opinion of Rambam, by leaving no space at the end of shema (or a space of less than nine letters) and then leaving a space of nine letters at the start of the next column before vehaya im shamoa.

Taz Orach Chaim 32:26 holds that one can attempt to accommodate the opinions of Rambam and Rosh by leaving a space of less than nine letters at the end of shema, and then leaving a space of less than nine letters at the start of the next column before vehaya im shamoa, such that the total of the two spaces is more than nine letters.

I am aware that this opinion of Taz has plenty of detractors, although it seems to be the fairly widespread ashkenazi practice today.

I understand, and Taz himself says, that this may not actually work to create a setumah according to the Rosh. But shouldn't this be fine according to Rambam, and therefore no worse than Shulchan Aruch's approach?

Rambam writes in Hilchot Tefillin 8:2:

וְאִם לֹא נִשְׁאַר מִן הַשִּׁיטָה כְּדֵי לְהַנִּיחַ הָרֶוַח כַּשִּׁעוּר וְלִכְתֹּב בְּסוֹף הַשִּׁיטָה תֵּבָה אַחַת יָנִיחַ הַכּל פָּנוּי וְיַנִּיחַ מְעַט רֶוַח מֵרֹאשׁ שִׁיטָה שְׁנִיָּה וְיַתְחִיל לִכְתֹּב הַפָּרָשָׁה הַסְּתוּמָה מֵאֶמְצַע שִׁיטָה שְׁנִיָּה. וְאִם גָּמַר בְּסוֹף הַשִּׁיטָה מַנִּיחַ מִתְּחִלַּת שִׁיטָה שְׁנִיָּה כְּשִׁעוּר הָרֶוַח וּמַתְחִיל לִכְתֹּב הַסְּתוּמָה מֵאֶמְצַע הַשִּׁיטָה.

This sounds to me like you only need to leave a nine letter space at the beginning of the line if the previous parsha finished at the end of the previous line. Otherwise, you can leave a smaller space (מעט רוח מראש שיטה שניה). Isn't this exactly what Taz is prescribing?

Answer 1

The Taz understands the Rambam you quoted to be saying the space at the end of the line and the beginning of the line combine into one gap.

The other understanding of the Rambam (which for various reasons is often seen as more likely what was intended by the Rambam) is that there are two factors: a space of 9 letters and, for a "closed" break, not starting at the beginning of the line. 5 letters at the end of a line and 5 at the beginning (which is what the Taz suggests) do not combine, and what you're left with is no parsha break at all, just a slightly irregular margin.