If it were me, I would mark them on the back with permanent marker and then cover the marked areas with the clear plastic you put on your rulers so that they don't slide. this would keep the marks from rubbing off as you use the ruler. Make sure that you use a brand new marker when you do this so that your lines will be as crisp as possible. You might even consider making every second line a different colour. That might keep it less confusing when you are using the ruler.