This topic will be alot of debate. I prefer to set up airplanes that will require little to no trim change at any power setting. The trick here is to get the wing, stab and thrust angles correct and then work on CG. With a flat bottomed airfoil, I set the wing to -1 degree, with semi-symmetrical I set them to 0 degree and symmetrical I set to +1 degree. The stab and engine are set to 0. Don't assume that an ARF has everything set correctly. Many times they do not. Now if you find your model trims at full throttle but dives at 1/2 throttle, you need to move CG back 1/4" at a time untill it stops. Keep this within reason and never move the CG back more then 35% of MAC. I'm sure I will be debated on this as I was on a different thread of the same topic. The issue was that a model was landing faster then the poster wanted and I had suggested moving the CG back a tad.