My personal preference is mid-lower knee, probably covering the knee (just barely).

Interestingly enough, the 'What would the Victorians think?' thread has some kilts riding at this length midway through Page 2 (http://www.xmarksthescot.com/forum/w...49/index2.html)

Of course I'm not so much a modernist anyway - I'd wager the kilt has covered and uncovered many knees in it's time.