The Bible defines nakedness as exposing the thigh (Isaiah 47:2-3). Since the knee cap is attached to the thigh bone, I think it counts as part of the thigh. This is why we teach that members of both sexes ought to cover their legs well below the knee, in such a manner that the knee would not be exposed under any normal circumstances. This would mean that a skirt should be at least mid-calf to play it safe. So as not to have a double standard, we tell the men that their pants ought to come to at least mid-calf. On the other hand, if they want to really be Christlike, they ought to wear long pants, since Revelation 1:13 says that Jesus was "clothed with a garment down to the feet."