OK, remember your high school math?? When you start to get into HSTs, you have to realize that you need to add 7/8" for a seam allowance instead of the standard 1/4" we become so accustom to. If you need to make a 9" HST, you need to cut two squares 9.875" So most quilters just add an inch and then sliver trim down the block. it may take a little more time, but you will have an accurate block.