I am not good at explaning another persons way. This was a online thing I found. The way I see it you are just finding the CG the airplane has. The airplane needs to be level ( I do not see a measurement needed for that) and when this is done as shown...that is the point of the CG you have. As I look at the photo...I see the more weight you add forward it will make the plumb bob move forward, showing you how much weight you need to move the CG a given distance. I know you will want too fly the plane and check more...like rolling into inverted flight and see if it climbs or dives.

One thing I liked is once you have the device you could easy use it on all your airplanes and maybe some of your friends too. The cost is not that much....and you could use it to store one airplane up and out of the way! Best Regards Capt,n
Note...if you add say 3 ounces to the engine end...the aireplane,s nosewill drop...then as you rotate the dowel with the string around it and get aircraft level again...you will see the point of plumb bob move forward.