It was always an adventure for me to get the 12 1/2 inch block. I have found that when I press the seams, I need to first set the seam with the iron and then press it open. I know it takes a moment longer to do the pressing, but it saves me so much agonizing afterward.
If all your blocks are the same 12 3/8 is not an issue. If they are not sometimes we just start over with that one block and then we add some pretty borders and make a doll quilt out of that block that is a little smaller and give it to some little girl who will appreciate it. I have done that many times -- to the point that when we have white gift Sunday at church they look for the doll quilts that they know are coming from me.
amh