Other things being equal, if the engine goes higher up on the firewall, a down-pitching moment will result, calling for less down-thrust angle (or more up-thrust). The theory is that the engine's vertical thrust line (axis of the propshaft in the vertical-longitudinal plane) should go through the center of drag of the model as a whole for stable flight behavior. The only practical solution is to fly the model straight & level at high throttle, then chop the throttle and watch for change of pitch up or down. Ideally, there should be no change, just slowing down into glide angle. Confirm by flying same pattern at low throttle, then punch up throttle. Only change should be in the model speed, no change in its line of flight other than lifting instead of gliding. Then for maximum enjoyment of the trimming experience, do the same thing inverted. If change of throttle results in change of pitch attitude, your vertical thrust offset needs adjusting. If you're moving your engine up the firewall by only maybe 1/4" the difference in flight behavior could be hard to notice, therefore negligible.