Great progress!

You might want to put a doubled bath towel over the table where you're ironing. At the heat you need for heavy material, you can damage the table. You might not care about that, but the softened-from-heat finish might mark up the outside of your kilt.

I thought you were going to artificially increase the rump measure so your aprons will hang straight? But you are still presenting measurements of 50 rump/51 waist, and calculating your pleats to that.

Here'e another method, at the risk of adding complication and confusing: Wearing p@nts, measure from side-seam to side-seam around the back of your rump. Then measure from side-seam to side-seam around the front of your waist. Add those two numbers, and make that your rump measurement.

Now measure from side-seam to side-seam around the back of your waist. The difference between the two measures in back (rump half to waist half) is how much you'll need to taper the pleats. Put all that taper in the pleats in the rear of the kilt, and don't taper the pleats that are near the apron (first 2 or 3 on either side of apron).

Does that make sense to you? I do hope I am not just making it harder. But if you try to build a kilt with 50 rump / 51 waist, I don't think it's going to turn out well.