Setting up throttle servo
The simplest solution I have found with a computer-controlled radio:
1.) Manually set up the throttle the same way you set your other servos, i.e., center the throttle servo to the center position of your thottle arm (I eyeball the hole in the carburetor to about middle).
2.) In most of my configurations after I complete #1 I need to adjust my high and low endpoints to some point below 100% -- with my trim in the middle I look for just slightly open.
3.) I refine the above by adjusting the control rod manually until my endpoints are equi-distant, e.g., high endpoint is 70% and low endpoint is 70%.
4.) Now you can use the radios's sensitivitie's adjustment (expo) to modify your throttle curve to the way you like it. I strive for linear control -- I keep adjusting the expo until it sounds like my throttle goes from idle to full in a linear fashion.
It's worth the time and effort you put into this setup.