This is a bit late now I’m sure but I’ll give you a pic and way I measure. Seems to work well for me and doesn’t have to factor any shrinkage per pleat, just over all. First I decide if I want a stitch on the seat center line or the stitches to straddle the center line. For this example well use the stitch on the center line. So what I do now is find the center of the piece I’m sewing and mark that line. Then on both sides of that line I mark .25”. This may be too small for you, if so mark 3/8 or a 1/2. Just whatever your comfortable with. Now we’ll work to the right over the original center line we drew. Well call that line number 1 and the other lines on the side of that number 2. So the number 2 line to the right of the number 1 line. Measure out how wide you want your pleat. I made my seat 4”. So I marked 4 inches over. Made a line then mark 2 more lines to the right of that in .25 increments. Then from the new far right line, I measure 4” again and 2 more lines in .25 increments till I don’t need anymore pleats to the right of the original center line. The opposite is the same when going to the left of the original center line. So every 4 inches I’ll have a set of 3 lines. I go to my machine and sew the center line of all the groups of 3. Once I’m done with that, I’ll fold my material on that stitch line. Now you’ll see 1 of the other lines of the group. Now sew on that second line of the group of 3. Repeat the whole way down. Now no matter what allowance you choose per pleat, the end result will be the 4” or whatever you want. The end pleat is always in this case 4.5” wide. I do this cause if for some reason it shrinks overall more than I wanted. There is seam allowance, and I can just cut out an individual pleat and just add it to the end or both ends.
