Hear is my rule of thumb. Use the minimum amount of control surface travel to accomplish the desired result. Use the maximum amount of servo travel (rotation) to accomplish this. This does not apply to the rudder in a pull-pull set up however. The idea is to give the servo the maximum mechanical advantage. and yet have enough travel to get the desired roll rate, loop size etc. The same applys to 3D set up's but often requires the use of larger servo arms to achieve this. Hope I havent confused you.
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