WHen you have the ruler set with the 45 degree angle if your block is going to be cut 8 1/2 inces put the center mark on 4 1/4 inces an trim two sides, then turn your ruler around and then set the ruler for 8 1/2 inches again putting the 45* angle thru the center and trim again. You block should be the proper size. No matter what size you want to end up your first cut you always center the ruler exactly half of what the final size of the unfinished block will be. Marge