I don't like math so reasoned it out in my head doing simple math: it's a starting point. For hst with a 1/2" seam add 1 3/4 to the square size.to get the diagonal seam to be 1/2" The starting size square would be 1/2" larger for the 1/2" seam.. So the square should be 2 1/4" larger. I would sew a test piece and be surprised if it is right and start over if it was wrong. LOL