The obligation to keep anything covered that is supposed to be covered (not addressing what areas might be included) has nothing to do with any shiur tefach.
Tefach b’isha ervah is the source for a man not being permitted to say kriyas shema (or any devorim shebkedusha) when his wife – and all the more so another woman who is ossur to him – reveals a tefach or more of what is supposed to be covered. Up until that tefach, he is still allowed to say any of the above.
It has nothing to do with how much a woman can allow herself to be uncovered.