It depends a lot on what block it is, how many seams and if there is a center element, etc. For example for a log cabin you could enlarge the center block 1/2 inch and then adjust the length, but not width, of the remaining logs. For a 9 patch I personally would enlarge each block by 1/8 inch and then fudge it so that you used a 12 3/8" block but that is probably cheating.
I've drafted blocks that were 12 3/8" finished for a special project but agree that 12 1/2" finished is an odd size.