ORIGINAL: retransit
Ah, but that is my point.
The plane must be level. The point on the hook is fixed, but the aircraft isn't. If you move the string to get the aircraft level, as it should be, you will see that the point of the plumb bob will move off your previous CG point. Try it yourself. If the plane is level and the plumb bob is on the CG mark, it is correct and you have equal moment arms on both sides of the balance point.
Of course you may wish to move that point fore or aft to obtain the characteristics that you may want in your aircraft. At least you will be in the ballpark for the first flight.
Graybeard, the hook works. A pulley would allow free wheeling movement and I prefer the friction of the cord against the hook to keep the aircraft from slipping to vertical.
Bob
When the plane isn't level, the place on the wing the bob points to wouldn't be correctly placed. retransit is correct. If you want the bob to point to where your cg is, the plane has to be level when you mark the CG on that wing.
When ANYONE refers to the location of the CG on the wing, they're really talking about where the CG's vertical plane passes through the wing. The CG is almost always above or below the wing.