The best way I have found to balance bipes is start out with the cg at 25% mac chord the overall chord of both wings. you allready have the cg mark then projct it to the top wing as you said and have someone hold there finger under the top wing on one side while you hold yours under the other. lift the plane off the bench and see how you are, move or add weight as needed.
This will work fine and will only take a second to do
hope this helps
Derf