Nativetexan - Just a guess here, but if you start off with a 4" square, add 1 1/2" strips all around, and for the second round sew on 4" strips, you should end up with a 13" block. Make another block with alternative colours, mark a line diagonally on one, place two together RSF and sew 1/4" either side of the line then cut down the middle. You should have close to 12.5" blocks. It's worth testing out with scraps first.