RE: Getting 1/2 stick to equal 1/2 throttle...
It's a combination of adjusting the throttle arm on the carb and adjusting the servo horn on the splines. Take the servo horn off the splines and adjust it 1 tooth either way.
Part of the problem is also with your EPA on the TX.
You want both EPA to be equal to each other--or as close as possible. For example:
The + EPA should be 80% travel
The - EPA should be set to 80% travel
If you can't get it right with 80%--then try 65% on both ends of the travel. This will give a much more linear throttle stick when you push it forward and back.
You'll have to move the servo horn a spline or two. And adjust the carb arm too. You may have to lengthen or shorten the pushrod 1/8" or 1/4"
It's just fine tuning. Everything you do effects the rest of the system--so make adjustments one at a time and check it.
Good luck