Just measuring my rump I get 19.5 - 20".
What is this measurement?

Personally, I don't see the point in measuring over pants. You're not going to be wearing pants under your kilt. Take your actual hip measurement around what you'll be wearing under your kilt, whether it's underwear or your birthday suit. Any adjustment from there can then be made based on a real measurement, not one that's been 'fudged' by random pieces of clothing.