How to correct CG on a not leveled aircraft?
You have a problem because you would have to know where the vertical center of gravity is to be able to mathematically calculate a correction factor to apply to the weighing points.   The higher the vertical C of G is, the more rear weight bias you will get if the plane is nose-high from level, or front weight bias if nose-low.  How much to allow for?  You don't know.

When the plane is level, it doesn't matter where the vertical CG is.  You would be better off finding some other part of the structure that is aligned with the longitudinal axis from drawings or the maintenance manual, typically something like an upper fuselage longeron, to level the airplane.