The kilt should fall to the middle of the knee when standing erect (not stooping) and should be clear of the ground when kneeling. Anything else immediately looks wrong. I once had a quiet word with an Englishman trying on a kilt which a shop in Edinburgh was going to be happy to sell him and which was a couple of inches below his knee. He didn't buy it. Kilt or no kilt if it is the wrong length it just makes you look like a cross-dresser I don't know why.