Bummer! I just finished one that had to be 12½ also. I did a trial block out of scraps and it came out about ¼ under. I did a scant ¼ on the real block and it is slightly over. I figured better over than under but some of my points may disappear. If it is for myself it wouldn't bug me so much because if all your blocks are repeated for the top, size is not as important as consistency. Good luck!