Hope I can explain clearly what I do by adding the sashing strips to square the block. If your block is to be 12 1/2" square, lay the block in the left hand corner of your cutting mat. If the block is too small lay it in the corner with equal distance all around to come up with the 12 1/2" square. Lay the sashing strips to the two opposing sides . making sure they are 12 1/2", pin and sew. Do the same to the other opposing sides. If you can't get enough of a seam allowance sew what you can and make it up on the next block that has had the sashing added. If the block is too big move the sashing strips in a bit and try easing in some of the excess of the block. I'm a very visual person, so if this doesn't make sense let me know and I'll walk you through the process.