I'm working on a storm at sea, but 1 block is 12 1/2 inches. I'm finding it challenging to get all my points accurate. One of the diamond squares in the one I'm doing is 3 by 6. I can't image what size is the diamond square in your 12 inch quilt. It's beautiful and now that I've been working on a full size I have a whole new appreciation for a mini. Unbelievable!