Does your controller have an analog axis that does not auto center? if you assign throttle to an auto centering axis, it will always return to 50% if you calibrate full motion of that axis. It would be better to assign to a non auto center axis.
even if you calibrated only 1/2 the axis (from center to max) you would still have the issue upon auto center, only this time the throttle would go to 0% or 100% depending if inverted or not.
you generally want to have throttle on a non centering axis for the mere fact that during flight you likely will set the throttle to some % and want to leave it there for longer periods of time.