You can't trim thrust angle by flying straight and level. You will simply compensate with aileron. Trimming thrust angle is one of the most difficult precision trimming tasks you can do. It take a variety of "tests" to really get it nailed down.

BTW, my GP Extra flew perfectly straight out of the box. But, during certain parts of an aerobatic sequence, it would not hold a line. I ended up adding over two degrees of right thrust before it would behave. I used techniques that the pattern guys use for precision trimming.

Good luck.