The source for regarding some part of the leg as immodest is a statement in the Talmud (Brachot 24b) that the "shok" is ervah (an area that should be covered). The basic debate revolves around the definition of "shok." Is it the area above the knee (the thigh), or below the knee (the calf)? The Mishneh Brurah holds that it is the thigh. Women who wear skirts that reveal the calf rely on his authority. Other authorities, such as Rav Vozner and Rav Kook, maintain that it is the calf. Each woman should follow the guidance of her rabbi and her community.