I like to make my HSTs slightly larger than I need and cut them down to size. I also ran across a product that isn't talked about as much called Stitching Lines. This product works wonderfully for me. I also use starch. Stitching Lines is simple a strip of paper that has the diagonal line and the sew lines marked. You take your squares right sides together and line up the strip on the diagonals then stitch on the sewing lines. The paper then tears away easily leaving your squares with the two sewing lines sewn. You then cut the square in half diagonally. Your cut could be a little off but it doesn't matter because the sewing lines are straight. Press the HST open and square it down. This works better for me than any other product because you can work just with the squares and don't have to have two larger pieces of fabric to start with. I make many HSTs but I don't always have larger pieces of fabric to start with (ie fat quarter). Stitching Lines can be ordered from their web site. The site is
www.marinandcolusa.com. I have seen them in quilt shops but not very often. If you contact them, they may send you a sample free.