I wear mine mid-knee, and usualy try to keep my kilt hose the width of 4 fingers below the bottom of my kilt. This is what i was told was 'proper'

That said, when i'm wearing my utilikilt or either of my 2 amerikilts, i tend to wear those slightly longer, about half an inch lower to just cover the knee cap, because they look better like that, and you dont wear kilt hose with them. Not sure if it's right, but it just seems to look more proper for the non-traditinal type