I will tell you my approach to it with models, no calculus, I passed but did not enjoy the trip through it, lots of pain involved - I look at what other guys have used and copy a successful setup.

Back in the good old days (may they forever be gone) we used to fly with very inadequate pulsing servos. The torque out of them was so low that the centering rubber band or spring would stall out at the largest throw - there was no force available at 45 degrees of throw. However we flew the airplanes of the time which had smaller surfaces and they worked OK. Even the smallest HS-50 has more torque than we used in those old pulse rigs.

The standard $12 modern servo by Futaba, JR, Hitec, etc. will work fine in normal large airplanes. I had them in a 100 inch span G38 gas powered Robin Hood - a high wing Curtis Robin semi scale model. They worked fine. I also use them in a Great Planes Ryan STA-M with 80 inch wing span.

However when it comes to the fire belching 3d monsters those guide lines are no longer valid. At that point I would look again to what the top guys are flying and then back off a little in cost.