Here is my rule of thumb for finding CG:

IF I were given a plane and had no idea where it should balance, I would balance it on the spar, but make sure it sits slightly nose-down - Because a nose heavy airplane will be sluggish, but it will still fly - A tail-heavy plane will be uncontrollable.

Then fly it.

Once airborne, First thing is to see how it handles. If it handles ok, I'll try a spin. If it won't spin, that's usually a good indication that it is nose-heavy.

Next comes the landing. If it won't slow down, that's another indication that it is nose-heavy

So if I have determined that it is nose-heavy, I will move the CG back in small increments (1/8 - 1/4" at a time depending on how bad it was the first time) until it flies the way I want it to.