I like to have as little bulk at the waist as possible for the kilts or kilt-skirts (5 - 6 yard) that I make for ladies. I measure from the natural waist (as Tess describes) down to the desired length. Instead of a 2" rise, I add 1/2" at the top for the waistband seam and at least 1" at the bottom for the hem, if the tartan is light weight.
I sew the apron strap at the waistline level with the buckle tab at the same level on the right hand side of the back of the kilt. I don't make a strap hole.
Then I use a large snap to secure the inner apron.
For some ladies, with a bit of a tummy, I make small darts not too far from the apron edges. That's not done for traditional kilts. I try to make them as inconspicuous as possible.
The other thing that is sometimes needed is a slight dip in the center of the apron...maybe 1/2" or 3/4". Instead of cutting to make that curves dip, I fold it over and enclose it within the waistband.
Here are photos of a knee length kilt-skirt and a mini-kilt.