You don't need a special ruler. There may be a problem if you are measuring in from the cut edge - it very probably is not straight. Start by measuring up from the selvage (be sure to take into account the selvage size)in a few spots. Until you feel comfortable, mark this measurement rather than cut it. Then using the ruler along the selvage edge (which should be straight) mark a line up to the 5 1/4" plus selvage mark. Now using the selvage, mark a spot 5 1/4" along the selvage . Now mark up (again using the ruler for the right angle) up a 5 1/4" plus selvage. Now connect the lines. Step back, take a breath and now remeasure your square. It should be 5 1/4". Now your confidence is back and go get that rotary cutter!!!!! (I might try this on some other fabric until you feel good about it.)