I do not think that you will be able to add measurements. But you may be able to add an X that goes from corner to corner to let you center it. You will need a heavy thread, such as buttonhole or topstitch, and clear packaging tape. I ran the thread from corner to corner with a bit of tension, secured it to the front with a bit of tape, then ran a piece of packaging tape on top of the thread. I will say that I only did this on a six inch square, don't know if it will work for anything larger.