I've many of the DNP quilts. I square up the blocks before I sew the nine patch and then I don't square up the cut blocks but sew them together and then square up the big block. It's a very forgiving pattern. I usually use 6 1/2 blocks and then gives me about an 18" finished block. I had a piece of pexi glass cut that is 17" 1/2 square. I square all the finished blocks using that so I end up with 17" blocks finished. Whatever size unfinished block I want I have that size ruler cut and then make sure all the blocks fit that ruler. It's so much easier then measuring. Having a pexi glass ruler cut is cheap. The 17 1/2" one cost me about $8.