I don't know if I can explain this very well, but, this is what I do. I find the center of the block size on the ruler I'm going to use. (in this case I would be using my 12.5" ruler. I mark with a black marker a dot at 3 3/4 inch. Line up my block with the center of the block and the black dot on my ruler. You will be cutting off approx a 1/4" all the way around. However I use the center to be sure it is squared up and is a 7 1/2" block when I'm through. Hope this helps get you started on the right track.