Well,
ideally you should just barely have to breathe on down elevator when the plane is inverted, you dont really want hands off level flight while inverted.
Theres really a process to properly trimming the plane that is pretty long and drawn out, you really need to start with step 1 and go through the process to get it right. Notice the chart says holding "A LOT" of down elevator while flying inverted. You should have a little down elevator while inverted you should only correct if it requires more than a breath.
Heres the trim Chart at NSRCA
[link=http://www.nsrca.org/TrimA.htm]NSRCA Pattern Plane Trim Chart[/link]



One of my friends I fly with and learn from who came in 3rd at the NATS in Advanced likes to have just a little TINY bit of down elevator trim in his planes when he is finished trimming. I had the same thing in my Prophecy and it flies really well in uplines, downlines and inverted.



I reckon that if your plane has a fixed wing incidence then all you really have to work with is CG and thrust to get it as close as you can...