Generally you set the throttle trim to the high trim setting (towards the top of the case) and adjust the throw so that the engine travels the entire range. The low engine trim is used then to kill the engine at low idle. Unforturnately, the basic 4 channel radio does not give you the option to dial the throw in from the transmitter. So you have to do it the way we did 25 years ago, trial and error. You can adjust the total throw by where you connect to the servo and throttle arm, plus by rotating the arm on the servo you can cut the total throw. Get it close for now, and you can always use the throttle trim to lower it enough to land.

BTW, the Arrow is a very good flying airplane. Good choice!