This thread is very informative about servos and math torque but I think a very direct way of knowing what kinda servo with how much torque , can be done by simply measuring the actual torque with a spring guage! They're easy to build using a spring ( and if you dont have a reference scale for calibration you can always take it to the supermarket and use their scale|) or bungee cord! Cheap fishing scales are usable as well and most of us probably have those in the garage behind the net! ..... Add in all the known payload..ie A can of Moosehead Beer(micro brewery) or the glass of wine for the Ladies.! Its the old ..which came first, the chicken or the egg(egg actually) ...build the arm or buy the servo first