Block it! I have often been able to eke out the 1/4 by blocking. To block your coming up short block this is how I do it. I have a cork bulletin board that I drew my 12 1/2" square on with a permanent sharpy marker using my square 12 1/2" ruler. I pin my block to it first doing opposite corners then the sides matching up the edges of my block to my marked outline on the bulletin board. You need to stretch it a bit but usually this is not a problem and I use a lot of pins to do this. Then take a spray bottle with water and spritz the pinned block until it is thoroughly saturated. Walk away and let it air dry overnight. When you unpin it it will retain the squared size of 12 1/2".