The Crow, sorry about our goofing off. We like to have fun.
No harm intended.
If the CG is correct and the stab and wing are in their proper orientation, trim the model at your favorite flying speed. That way when you decrease throttle it will slowly descend. Add power and it will climb.
Trainers have a natural tendancy to climb. That is the way they are. You may want to consider moving up to a mid-wing or low-wing aerobatic trainer for a little more stable performance.
Years ago, my instructor, gave me a very important bit of advice: the KISS method. Keep It Simple Stupid! It works. The more you alter the way the canooter valve operates in relation to the kadutzit pin the more variables you throw into the equation and the greater the likelyhood of something going wrong. The frammish pin is a whole nother story.

Best of Luck,
Jeff