Typical method for dealing with bigger rump/smaller waist: imagine you're wearing p@nts and measure from "side seam to side seam" around the front of your waist and around the back of your rump. Add those two numbers together for your finished kilt rump measure. That way, the front apron will hang straight down from waist instead of trying to "pull in" at the rump. If you made the rump to your actual, smaller measure, the front apron would tend to "pull in" at the hips.

Glad to be of help and keep us posted on your progress!