I put a thin piece of masking tape parallel to the wing at the point I've calculated and then use my finger tips. You can feel the tape, and it makes it easy to tell where you are in relation to the measurement.

In years of doing this I have found many plans that have extremely conservative CGs. Determining this before setting up the airplane keeps things simple, because I'm not trying achieve a balance point that is farther forward than I need.

Then I do flight testing. I rarely find that I need to adjust the CG once I have calculated it. Out of the last dozen or so planes, I moved the CG further back on one; on another I moved forward, not for pitch stability but for yaw control, but that is another story.

Jim