Waist and seat measurements are easy to make, and they tend to be repeatable.

Kilt length can be an enormous bugaboo. I did my best a few different ways, got to repeatable results, and my kilt maker said I was asking for a kilt length apprpriate to someone about 6'4" tall. I am 5'10".

I fixed the "problem" by ordering an inexpensive poly kilt, put it on, took a picture, posted the picture here and asked if the length looked about right.

I ended up with a tank from Scotland that fit correctly.

Since you already have a kilt or two, you could put it on, put the hem where you want it, take a pic with your hands on your natural waist, post it here and we can go from there.

Nothing sucks quite as much as spending hundreds of dollars on a made to measure tank and then have it come in with the wrong length. They can be hemmed if too long, but there went your swish...