First decide if you want to wear your kilt as a traditional kilt (at the natural waist) or as a more contemporary kilt, at the pants waist. For various reasons, I highly recommend the natural waist.

To get the length, kneel on the floor with your thighs and torso straight up and down. Your kilt should just brush the floor. Your natural waist is about at your bellybutton anyway, so use it as a reference...wrap a measuring tape around your torso at the bellybutton, note whether the top or bottom edge of the tape is at your bellybutton, make sure the tape is level front to back and side to side, take the measurement, and mark the "bellybutton line" on your back. Now have someone measure from that mark to the floor, as you're kneeling. Add two inches for a two inch rise, add four inches for a four inch rise, etc., and that is the length of the traditional kilt that will fit you. FWIW, my kilt has about a 1" rise, as measured. Not a bad set of measurements to have in mind, if you're at a Games and browsing the OTR kilts! A kiltmaker will need more measurements than that, but as far as length, the kneeling measurement from the bellybutton reference will get the job done perfectly.

IMHO, trying to do it with a towel is sketchy at best, particularly if you've never worn a kilt before (you may end up looking like some of the ignominious and hilarious "kilts" in the Media section...we should have a thread for that, ie "These are NOT kilts" ...) , and only results in a continuation of IMHO unnecessary discussions about whether someone is wearing their kilt too low, too high, etc...measure it correctly, wear it at the right level on your torso, and you'll not need to worry about fabric sawing at the back of your knees (because it will, if you're sagging your kilt like a gangsta), nor about flashing your thighs like a schoolgirl.

Later, if for some reason you either need or are forced by social convention to wear your kilt at a slightly different height, you can always hike it up (for water crossings) or down (to avoid scandalizing the congregation with your dead-sexy knees) as necessary...it only takes a second. When you're actually wearing it, it may migrate up and down through the day, but my kilt doesn't generally shift more than an inch downward anyway, and that's after hours and hours of wearing/walking/sliding on the back of it down a snowfield/etc...

So yes, it moves, yes, there's a range around your knee where it looks "best" IMHO, and if you measure correctly in the first place for the style of kilt you want (pants waist vs natural waist), you're more likely to get "in the ballpark" when it comes time to kilt up.

Sorry for the digression, you were asking about St. Kilda kilts...

-Sean