Only other option I can come up with if Knitette's doesn't work for you, find a way to mark at least a small intersecting line in the center of your block and line up your ruler on that mark in both directions to square up. Fold the block in half; finger press; fold in half the other way; finger press to find your center. Even if you marked on the back and squared up from that side so you didn't have to worry about the marks????