The pleats on a kilt made in accordance with "The Art of Kiltmaking" will be between 5/8" and 3/4".
Any larger than 1 1/4" and you are up in a cargo kilt range. A Utilikilt brand garment will have varying pleat widths between 1 1/4" up to 2 1/4".
The apron at the waist will be a bit wider than 1/2 the waist circumference.
The apron at the hip will be a bit less than 1/2 the hip circumference.
About Cotton - People have been trying to find ways for cotton to hold a crease for a lot of years. The best solution is to blend the cotton with a synthetic fiber and that is why we see Poly/cotton fabrics so much today. When you set the creases in Poly/cotton you are actually melting the Polyester component.
The only other way is to edge stitch.
The vinegar method will help but will come out after just a few washings. The idea of a 'chemical set' is actually more of a softening of the fibers to allow them to be pressed easier.
You say that your fabric is heavy brushed Twill. Do you know that actual weight? I've never seen cotton twill woven in Tartan at anything more than pajama bottom fabric.
To find the weight of your fabric you weight a piece of your fabric 60" x 36". This will be the weight in ounces of 1 fabric yard. The average kilt fabric will be between 13 and 16 ounces per fabric yard.
Thanks so much for the info.. stoked I've been using good measurements on the aprons and pleats. As for the weight of the fabric I'm not sure. It's definitely heavier than shirting... It was labeled as medium weight. I know that doesn't say much for cotton lol. Too late to weigh it since I cut it, but it's only available in 45" width. It's settled in going to edge such this one as well.. as it was said better safe than sorry after all the work. 🤔