A good way to get into the ball park (probably + or - an inch or less) of your "correct" length measurement is to wrap a towel around your waist about 2" above your navel (or at whatever height you want to wear your kilt).

Then, ask a friend (preferably someone who knows how to sew, not just any of your drinking buddies) to pin the bottom of the towel up to the level where you want to wear the kilt - above the knee, at the top of knee, middle knee, or somewhere in between. Stand up straight and tall during the pinning. Do not look down or you might perturb the measurement.

Then, simply take the towel off and measure from the top of the towel to the pins. That will be very close to your actual length (at least within the "ho ho" test).

To get your actual length - the one you want to send your kilt maker, use any of the various measuring techniques described on the web pages of many of the kilt makers on this forum. The result should be close to your "towel" measurement. If not, seek professional help!