eparys

Just checking back in to see if you had responded - sorry that I misunderstood what you were trying to do. My only addition to all that great info that bearisgray and Iceblossom gave you is not to skimp on those triangles (imho). Depending on how you are setting the blocks, I tend to to try and cut them so that the outer edges of the corners are on the straight of the goods. That would mean using two 9 3/8 inch squares each cut once on the diagonal. That being said, I always cut them a minimum of 1/2 inch larger that what is called for (in this case about 10"). I match and pin the center points before sewing. This gives me a little extra wiggle room on the outside when I trim up the block.