R’ Moshe says a tefach cumulative. Being male, it’s completely academic for me, but I’ve wondered how it’s possible to have that “envelope pushing” that people talk about with hair covering. If going by the tefach rule it would take a serious amount of math and measuring to ensure that you were just within the bounds at all time. 😉
bpt: Your comment is another thing I never understood about women’s tznius. Why are leggings considered less tznius than tights since they’re so much thicker? Well, guess I don’t have to worry about it till I’m married and then I can ask my wife. 😛