A couple of generalizations -which may be handy

when measuring relative servo arm length and control surface horn length
measure from center (hinging point)
the servo arm length of aprox 75% of surface horn length is a rough starting point- for a good 3D setup
rudder/el/ ail -then readjust servo horn length (shorten it if possible). This isall about maximizing force available
at speed I have seen guys using $$$servos on 42% models complain and demonstrate aileron blowback at cruise speed -
Bigger models have far more airload problems (has to do with RN) and servos just simply may not be up to the task.
Iyt is hard to believe the 300"ounce stuf (3 per ailerons) cant do th e job - but seen it and the only fix is spades . just like full scale