If you are trying to make sure all of the blocks are the same size the first thing you do is yes find the center which would be at the 5 1/4; then make sure the top/bottom and both sides of the ruler are at the same 5 1/4 so you are "centered" and then trim anything outside of that area. You need to have a 10 1/2 or larger ruler to do it right. If you don't center it before you square it your block won't be even all around.