I've always balanced my planes from the prop. Not sure if it's the best way to do things, but since the prop shaft is axially centered I thought it should lead to more accurate readings. If you place a screw on the top center of the firewall it seems that you'll have a lot more weight below the actual pivot point and that will cause the plane to try and self-level itself regardless of what the balance really is. At least thats how I see it visually. I may be completely wrong though.