Halacha: tefach above knee is erva
Minhag: completely cover the knee
Chumra: 4 inches below
Shtus: I as a man a man or woman get to be the tznius police and determine everything else about this person based on how many inches her skirts are below the knee. I also get to say that if her skirt is just barely below her knee she is being nichshal the rabbim