If you want to start with a 13.5" block, each of the 4 half square triangles will need to be 7 inches. So I would cut 8 inch squares, make the hst blocks, trim down a bit to 7". When you join the two 7" blocks, you loose a half inch in the seam (two 1/4" seams). The result is 13.5". Not sure what rule of thumb you are looking for. Something about the pinwheel block or the recut disappearing 9 patch??