There are no set rules really, it's a matter of physics.

If you use the outermost hole on the servo, and the innermost hole on the control horn, you will get the most amount of throw. You will also be putting the most strain on the servo due to the amount of leverage the control surface has over the servo.

If you reverse the setup, (Innermost on servo - outermost on surface) you will get the least amount of throw, but your servo will have the strongest torque.

I usually start both on the outermost holes (It's rare that I want LESS control) and adjust accordingly. (In on servo for less control - in on surface for more control)