I have servos ranging from 300 in ounces torque to 15 in ounces
digital /non digital , on and on.
for just flying around with say a sport 40 powered Cub - and an occasional loop etc., a decent non digital servo -one each for ailerons , elevators , rudder (total 3) at 30 inch ounces of torque -is plenty That is from experience - no calculations - I never calculate -except against sales people -- I SWAG (scientific wild arsed guess) and it works very well -for me.
The recent explosion of huge power output servos has occurred along with huge 3D models and high output batts .
The old "40 in ounce servos were not bad but had lousy power at center -- this is where the latest chips and motors and good gear trains have produced a quite improvement -that fact has not gotten a lot of press
I recently got aholt (!) of some very inexpensive JR Sport mini servos MN48---they are rated at 48 in ozs on 4.8 v - In testing them - resolution is noticable on the model - with each click of trim on my JR 6102 Tx ! That is all that is needed.
My point is that high output servos ane NOT required now to get good power at reduced throws -where 99.99% of flying is done.
going to smaller servos -,I fly 15 in ounce servos on foamies with 100 wat motor setups and these will do fast and furious direction reversals all day long - BUT they do heat up doing those tricks and they have teeny weenie gears. So for just fun easy sport flying -- on 40 sized glow models - stick with mini servos -or larger from the better servo mfgrs .they are cheap ($23.00 street price on the 48's) Trying to calculate servo load may be fun but unless you are a real kluge at geometry and are terrible at linkages - this approach works fine.