How do you work out a seat from a kilt?

[KiltFitz]

We talked about presenting him with the cloth but there is an elaborate joke to be fulfilled which will only work with the kilt as made. It is for a made to measure kilt but that shouldn't cause too much panic: I've been working in highlandwear retail for a while now so I am reasonably confident I can get this right. I routinely measure customers for machine and hand sewn options of various styles and yardage.

I'm just curious what a craft kiltmaker would do if tasked with making a replica of an existing kilt. How would you determine its dimensions? It's not a situation I have encountered before. There is a reason I don't want to approach anyone at my employer for advice however.

For seat measurement I normally take a loose measurement while someone is wearing trousers across the widest point. This kilt is going to be made for growth anyway so there will be more than the usual range of movement for the aprons.