I have also found it helpful when measuring kilts for guys like this to take two additional measurements as a backup.

Barb has already pointed out -
The aprons are made from a flat piece of fabric and hang straight down from the belly. All the shape of the kilt is in the rear.

Well, the apron edges are flared and if you will look at the third photo Barb posted above you should be able to see that the apron edge hangs straight down.

What I do is measure across the back - at the waist - from apron edge to apron edge.
Then
Measure across the back - at the hips - from apron edge to apron edge.



The difference between these two is the amount you need to taper the Fell.

If you do not have a kilt to take these two additional measurements you can use the side seam of a pair of trousers or even a door frame or other vertical edge. Just something to give you a vertical reference.

These two additional measurements do not need to be perfect. They are a back up or confirmation of the measurements you got using the pillow/towel method.