I think I've seen a "formula" to correct that problem. If you need a 1.5" finished square, and you have to make half-square triangles, you cut your square 1 7/8". Konstantin may even written something about this, and I'm sure Penny Halgren has, too. But one of the easiest ways I've found yet to make HSTs is to put two 5" ot 6" (or whatever size you need) pieces of fabric together, right sides together, sew all around with a 1/4" seam, then cut diagonally both ways. Voila, four perfect HSTs. There's a video on YouTube by Missouri Quilt Co. on this neat technique. Their videos are the best! (Even I can follow them!)