Hanging from a string to the prop shaft and tail wheel will allow you to balance the plane in roll - you can tell if one wing is heavier. What you are experiencing with your balance setup occurs because the cg is higher than the points you are resting the plane on.This works fine, it is just a little tedious. If you turn the plane over it will be stable on the balance . If it is nose heavy, the nose will slope down when near balanced and will stay put . If you are way off it will swing way down. It is balanced when the plane rests level. I almost alway rest the bottom of the fuse on a small cross beam at the intended cg. You know it is right when a little added weight on the nose causes it to fall forward and a little removed wewight causes the nose to swing up. It is actually more sensitive than when the plane rests on supports above the cg.