Do you have a large square ruler? It is really helpful to have a ruler the unfinished size of the blocks (eg. 12.5"). Mark the center of the ruler (for 12.5", it would be at 6 1/4"). Place the mark in the center of your block and trim 2 sides. Turn and trim other 2 sides.
If you don't have the large square ruler, I measure out from the center, and then measure from the edge of the plate. If you plate measures 5" from the center, cut the edge of the block 1 1/4" from the edge of the plate. Turn the block and line up the cross lines on the ruler on the side just cut (to get you 90 degree corner) and 1 1/4" from edge of plate and cut again. Repeat for other two sides.