My favorite square-up ruler is a 12.5" frosted Olfa. It has a diagonal line, with little boxes marked on the diagonal. I put the upper edge and the right edge of the ruler on the block, then look at which little box on the diagonal line is closest to the center of the block. (98% of the time, the block has a clearly defined center.) Then I move the ruler so the little box is dead center on the seam lines of the block, and adjust so the vertical and horizontal lines on the ruler align with the vertical and horizontal lines of the block. Then I trim the upper edge and right edge of the block. Rotate the block and do the same thing with the other two edges.
Clear as mud?
