Whew! I'm glad that one is over. When I change the size of a pattern for a larger block, I just lay the 10 inch block piece on a piece of white paper and increase 1/2 inch around then redraw piece. Makes sense to me. A 10 inch block with 1/4 seam allowance + 10.5 add 2 inches to that and you get a pattern piece of 12.5 inches. Easier than all that figuring our.