“Shulchan Aruch Orach Chaim 75:1 talks about how tznious is dependent to your surroundings”
“On the other hand, the Mishnah Berurah does understand the Gemara in Berachos to be delineating a base line of what must be covered, and that is why shok is singled out (albeit with a more meikel definition of shok). While the Mishnah Berurah’s position has become the presumption in the Yeshiva world, one can’t say that it is wrong to rely on the Shulchan Aruch.”
The Shulchan Aruch says no such thing. He writes (75/1): “tefach meguleh b’isha b’makom shedarka l’chsoso, afilu hi ishto, assur l’kros krias shema k’negda”.
“If a tefach is showing in a woman of the places that are supposed to be covered, a man is not allowed to say Krias Shema in front of her”
The Mishna Berurah explains that the Shulchan Aruch is referring to:
1. the upper legs and upper arms EVEN if the minhag hamakom is for them to be uncovered.
2. The lower arms and legs if the minhag hamakom is for them to be covered.