Unless you want to cut some unusual fractions, you can't get it to come out to 12-1/2". If you start with 5" squares for your 9-patch, it will make a finished block of 13". Starting with 4-1/2" squares will get you 11-1/2" finished. Starting with 4" squares will finish at 10". You could go smaller and add borders to the blocks, or use any of the sizes above and adjust your quilt border sizes to make the quilt a certain size.