Measure back from the center most point on the top wing's leading edge, 3.1".

Make a mark UNDER the top wing at this position.

Hold the model there with your fingers or better yet with a pencil eraser on either side of the centerline at the marked point.

If the nose goes down, add weight to the tail until it's level.

If the nose goes up, add weight to the nose until it's level.

Don't go by projecting this point onto the fuselage.