Here's what you need:
http://home.mindspring.com/~the-plum...%20Machine.htm
You didn't do anything wrong. You adjust the plane so that the horizontal stabilizer is level. Then the plumb bob gives you the exact CG.
Two nice things about it: You can leave the plane hanging there all day while you move batteries etc. around inside. And you can balance a low-wing plane right side up. Try doing those things while holding a 20-pound plane up with your finger tips.
