Your problem has been realized because your calculations didn't take into account the thickness of the fabric and then all the innards that DO IN FACT take up space inside the kilt. On a normal sized adult, this added thickness is virtually un-noticeable. However, when you try to use that same process on a hip cirumference that's maybe 1/2 to 1/3 of what a normal hip circumference would be, you have to take that into account and go larger on the outside to make your inside circumference what you're trying to achieve.