This is one of those tools that you can easily live without, but it sure is a time saver when laying out a bunch of center points. I drilled a hole through the dowel for a tight fit with a round mechanical pencil. The pencil had just a little wobble in the hole, so using a Q-tip, I wet the inside of the hole to raise the grain inside the dowel. This made for a snug fit, without any wobble. After trying it out though, I found that the pencil lead would break off too easily. I ended up wrapping some tape around a standard #2 pencil, to make it fit in the oversized hole. This pencil works much better.
It’s important to drill the holes for the rollers, in line with the pencil tip, and also exactly equal distance from the tip. I used shower door rollers that use an 8-32 screw for mounting.