There has been alot of debate and articles about servo torque requirements over the years. My engineer son tells me it involves the aera of the control surface, the degree of deflection of the surface, and the speed of the plane. Something about fluid dynamics? There have even been programs in which you can plug in the specfics as to surface size and deflection and speeds and get recomended ranges for servo torque. Don't make it harder than it is, the mechanical advantage stuff is right on info and standard size planes are generally ok with standard size servos. If you go bigger or faster or 3D then a little more $ is a good idea.