The optimum length of the servo arm depends on the tail rotor bellcrank geometry oo your heli.

So this distance will vary from type of heli to another. The arm length that will work well with one, may well not be the ideal length for another. e.g. a longer bellcrank arm will require a longer servo arm (distance from the center of the servo arm to the ball on the servo arm).

So you may well fnd here that a shorter servo arm length then 14mm will be better, maybe as small as 10 with increased atv setting at the gyro pot (not in the TX)

Albert