Fon's and Porter is the tool you're thinking of. If you do not want to purchase one, just draw a line with a ruler thru the center and sew a quarter inch on either side of the drawn line. It works just as well. I was introduced to a pen called FriXion. one end is an eraser to rub out the lines, or It comes off with a Hot dry iron. It's made by Pentel.
It comes in three colors red blue and black. I suggest you give it a try. It costs less than the rulers.
AuntJudy14