Never heard of that one Dee...good idea.
Cyberpyrot - you don't do the bearing surface so your barrel never touches the sharpie mark, only the tip.
Also, part 2 of advice....it doesn't work worth a crap on brown paper, white is best. I learned this the hard way. Because while shooting a ladder test I have 5 different colors so I don't have to go down there as often. Well with brown paper I couldn't tell which was which.
Don't let your ears hear what your eyes didn't see, and don't let your mouth say what your heart doesn't feel