Depends on the airfoil, symmetric sections and you are trying to get similar responses upright and inverted. Won't ever be the same, the wing has to fly at the same AOA up and down, so there will be some trim diff either way, you are looking to minimise the difference without getting into pitch divergence. Cambered sections need the '30deg down' trim thing, ie trim for 'S&L' then without changing the elevator trim, push the nose down, allow the speed to build, then release the controls and watch the model's response. You are looking for a gentle pull out from the dive. Sharp pull up = nose heavy, tuck down = tail heavy.
Evan, WB #12.