When you balance the plane, you want to balance it on the wing. The plane is literally hanging from the wing.

While what you're doing will achieve the same result, it's going to be a lot more difficult to get an accurate measure of the CG location. The main mass of the plane is above the point on which you're balancing it, and it will be a lot less stable. It'll want to tip very easily.

Balancing the plane does not have to be so sophisitcated. Your fingertips will work fine. Mark the balance point on the underside of the wing on each side of the fuselage. Place a fingertip on each mark, and lift the plane.