Thats it - the proportion is what is important to get right.

Having seen a man with a pronounced 'bay window' wearing his kilt under it, and the hem was slightly above the knees - the effect was rather disasterous. It looked untidy, ill fitting and most unfortunately, comical.

You need the kilt long enough to look right, and with the hem no lower than the knee joints the only way to get more length is to go up.

I'd advise that the hem be at the knee joint, not the top of the knee cap for anyone with a bit of a bay - to make that bit more length for the kilt, and the illusion of more height, less width in the body.