You really can't get good measurements for a new kilt from an existing kilt (except for length - see below, though). For example, many kiltmakers, including me, add an inch to the right hand apron edge to make sure it covers the underapron. The extra on the apron edge is necessary because a kilt has a finite thickness, and, if you made it exactly to someone's measurements, it wouldn't go quite all the way round what with the pleating, the canvas, the lining, and the double thick aprons. I also add an inch to the underapron right hand edge at the hips, and a half inch at the waist. This is so that the underapron can come far enough around for the strap to reach and go through the "buttonhole" where the strap goes through to the outside of the kilt - it's typically an inch or more back from the edge of the apron. If you didn't add a little, the strap wouldn't make it far enough through the buttonhole to buckle comfortably. (And you add 1/2" less at the _right hand_ underapron edge at the waist than at the hips so that the waist really snugs in and lets the apron buckle over top, which is why you don't add at the left underapron edge instead).

For all of the above reasons, if you measured an existing kilt and gave the waist and hip measurements to a kiltmaker, you could easily wind up with a kilt that is too big.

It truly isn't hard to measure waist and hips for a kilt. Get someone else to do it (don't try to do it on yourself), and use the measuring instructions that your kiltmaker provides.

The hardest thing to measure is length, and again, you should use the exact measuring instructions that the kiltmaker asks you to use. If you don't, you can wind up with a kilt that is 2" too short, 2" too long, or something in between.