I recently saw a demo on making four HST. She took two pieces of fabric, right sides together. Then she stitched 1/4" all around the four sides. She cut from corner to corner and then corner to corner (or an "X"). When opened, she had four identical HST. WOW!!
The down side of this technique is that the outside edges are on the bias, so handle carefully! Stabilize against a straigh-grain piece.
Suz
PS: I don't know how much to add to the finished size, but I am guessing 1-1/4". Anyone know for sure?