About every other month we go through this very problem and there are numerous posts as well as compete threads on making throttle adjusements without using the TX End Points. You can search on Transmitter Linkage or Transmitter End Points and find the applicable threads.

Transmitter end point adjustments are the LAST thing you should be playing with. The first thing to do is to adjust the mechanical linkage to get the best linearity and travel out of both the servo as well as the throttle arm without going to the mechanical end stops. That way you will get the best resolution and prevent binding of the servo. Once that is done, you can make fine adjustments to the throttle... that's FINE ADJUSTMENTS, to the throttle channel.

None of the initial adjustments and settings should be done with the electronic end point settings on the transmitter.

CGr.