I had 1 inch to the finished size I want the square to be. For a HST that needs to be 3.5 inches, I cut two squares at 4.5 inches. I sew 1/4 inch on each side of the diagonal, cut and press. Then I square it back up.
There is a bit to trim, but I never end up short. Maybe when I'm a grown up quilter instead of a Wanna_B I won't have to allow so much fudge room.