I have to be right on for a swap. I've been doing the Craftsy BOM (free) and all my blocks except 1 were exactly 12 1/2. One had one corner off by 1/16. I wanted to redo it, but when I tried, the block turned out funky, so I kept the original and put a pin in the short side so when I go to put it together, I'll know to adjust it so it will fit.