Man, I remember that feeling. When I ordered my first kilt--it was before such things were done on the internet; it was done by mail--I must have spent at least two months building up the courage to actually send it off. I was worried about the correctness of the measurements, etc.

You'll either have to finally just take the plunge (I'd go just a little tight on the largest size; maybe an inch) or do as articifer suggests and get two PV kilts. (I don't usually advise anything other than wool, but you are quite worried about the variation in size.)

If you don't go ahead and do something, you won't get kilted at all.