Originally Posted by tdearth
It would also be nice to have reference bumps or dimples at the CG on the top of the wing. Because of the mass of the motor above the plane it is easier to balance the plane upside down.
I agree that upside down is the way to balance. Here's how to to it properly, as demonstrated by Lockey with one of his Depron creations:
Or you can use your thumb and finger!
As for locating the CG, it's easy if you note that the rear of the vent is directly over the step.