If its a "Pattern Style" model www.nsrca.org has a trimming guide.

The guide assumes a 0-0-0 (0 deg downthruts, 0 deg wing incidence, 0 deg stab incidence) symetrical airfoiled everything in a straight line setup. If the model deviates from the setuyp... its usually in wing placement. airfoil and "decalage" (angle between main wing and tailplane incidences) that will throw the trim chart off. These being different usually void the dive test, and can void the inverted flight tests.

If you have an angle between the main wing and stab ncidences.. you will 99.9% of the time need down elevator for inverted flight. (about 90% of the stick travel for a typical trainer...) moving the CG back MIGHT be the answer for your model... but I'm not familiar with your model.