My guess would be that the blocks are 12x12 finished, so the unfished size you would trim to would be 12 1/2 x 12 1/2 just like your ruler. Lay your ruler down over the top of your block and line it up so that the block is straight up/down and side/side. Then you would trim to the ruler dimensions. Sometimes you have to fudge a little if the block isn't big enough or if there are points along the edges. Can you post a picture of the block? That may help us help you.