Nice question but the answer is much bigger than you might expect.
The servo output arm, pushrod, control horn and control surface are something that you need to consider as something serving a single purpose and there are many things to consider as much of the setup is dependent upon the setup of the other components.
Firstly you will need to know how much control surface deflection you are trying to achieve and the force required for the deflection.
Next to consider is the strength or the pushrod or rather its rigidity as well as its straightness and length.
What control horn clearances and size
Clearance or available room for the servo output arm to operate within.
The factors effecting the whole design or setup are the required control surface deflection, the travel range of the servo output arm and the rigidity of the pushrod. Consider the pushrod flexibility and its effect upon the whole setup. If it is very rigid the pushrod will flex very little of even not at all and so it can handle short but powerful movement ranges. If the pushrod is light and a little flexible then it would be preferable to use long and less powerful movement ranges so that the compression forces are lower and any flex will have less effect in varying the position of the control surface.
Once you have determined the amount of pushrod movement you want for your design and the control surface deflection you require you can start connecting things.
With a lot of pushrod movement, (weak or flexible pushrod), I start out with the pushrod connected into the control horn in the hole closest to the hinge line of the control surface and conversely where I have a short pushrod travel I use the hole in the control horn furthermost from the control surface hinge line.
With the control horn connected and knowing the required deflection for the control surface I then fit the other end of the pushrod to the servo output arm required to give the correct deflection range for the control surface.
As a rule I try to keep my pushrod movements with a little flexibility but not too much, (helps reduce servo gear stripping on impact, note helps reduce NOT stop all together). Other installations where I would recommend large amounts of pushrod movement are installations where the pushrod has a bend or curve which is used to exit the pushrod from the fuselage.
As for my control horn and servo sizes t tend to use sizes that are in keeping with the overall size of the model.
I hope this doesn't leave you more confused than before but it is just a few of the considerations and thought processes I use in my designing the control setups for the models I design and sell.
Have fun!
