I really don't like the kneeling method. I have had good success measuring myself on kilt length using the following method. You have to do this looking into a mirror and standing up tall.

Tie a string or put a rubber band around your knee where you want the kilt to fall to. Holding the "0" end of a tape measure, pull it up through the string along your side (making sure the string doesn't move) to where you measured your waist (or in Rocky's example, where you want the top of the kilt to fall). You can then lean over and read the length at the string. Do this several times to make sure your readng is consistent.