ORIGINAL: Bozarth
A few pictures are worth a few hundred thousand words:
Kurt
Picture 1 is good. You will actually be able to mark where the CG is on the wing.
Picture 2 is not. The mfg said the CG should be X inches back from the LE. Where is the plumb bob pointing? Where would it be pointing if the plane was actually level, which is how you measure the X inches back that the mfg suggested. And Picture 2 and 3 really screw you up when the airplane wing isn't exactly centered in the fuselage. A high or low wing plane in the Picture 1 and Picture 2 positions are going to be impossible to guess at where the CG would be back from the LE of the wing.
Picture 3 is not. It's just as hard to guess where to mark the CG as in picture 2.
You really will not know much of anything when your plane isn't horizontal other than the CG isn't right, and which way it's 'too far'. Good thing is that you can tape weights on the sucker while it's suspended to find out how much weight is needed where. If you were crafty and did the test before installing some heavy components, they'd be taped on the sucker and you'd simply move them until the plane was horizontal. THAT'S the most efficient way to use that rig. Then Pictures 2 and 3 would be what you'd see just before you saw Picture 1.
Picture 4 is what it is: something you can't see with the rig, and something you can see.