For my own kilts, I measure the back of my waist
from where I want the pleats to start to where they
stop. I do the same for the hips. The difference
between these measurements is how much I
make the pleats taper in the fell.

For the front I measure around the entire waist.
What isn't covered by the pleats like I described
above is made up with the apron.

I could measure the front using the same method
as I described for the back, but it is too easy to
not get the tape measure on the same exact
spots, and have a wrong size kilt.

By the way, the reason I do it this way is because
my waist sticks out in the front as much as by
butt sticks out in the back, giving both the
waist and hips the same measurement even
though I am not cylindrical shaped.