I've secured an older EXTRA 230 (built from Kit) removed all components, checked frame integrity, addressed any covering issues and reinstalled new engine, fuel system, battery, servos and receiver...

My question the former owner didn't have a user manual so I do not know how to determine CG. Is there a formula?

I was going to use the point on the wing where the rib is thickest as the CG point. Am I way off?