I found the best way for me to make sure my block is going to come out to 12.5" is to grab my graph paper. I take the block I like and figure it out ina 12 x 12 square. Then I add my seam allowances to the number of squares each piece is. ( I don't know if that came out correctly or not!)
But what I have discovered is that I love my graph paper.
Kendra