The basic rule is to add 7/8" to the size of the desired FINISHED block. Eg if you want 3" FINISHED block then cut your blocks to be used as triangles 3 7/8", 6" finished blocks cut fabric 6 7/8". Then draw a line down the diaginal of the block and sew 1/4" on either side of that line. Cut on the line and Voila 2 HST. After cutting the fabric you will get desired FINISHED block size + 1/2"