True, the CG is a point on the front to back line of the plane and is not dependent on the plane's attitude, but it's impossible to check if the balance point you've chosen is below the vertical CG of the plane. On a low wing and many mid wing planes, it is impossible to get a plane to balance with your fingers or device on the bottom of the wing. You know you're doing it the wrong way when the plane tips either forward or backward no matter what you do and will never just balance. So pick the method that works. If the plane will balance stable when it's inverted, then that's the right way. If it will balance stable upright, then that's the right way. It's really not any more complicated than that.