[EricH]

I'll add a little to what Tom said--those formulas (and the definitions for PD and PA) can be found in a machine design textbook (check a college bookstore, or somewhere similar).

One other thing to be concerned with, at least a little bit, is the width of the gears in the gearset. Wider gears can be less susceptible to breaking (due to somewhat lower bending stresses in the teeth), but obviously weigh more. Any 2 mating gears should be the same width--not doing that increases the stress in the wider gear.

General process: Find the total reduction, then figure out any intermediate steps (may be forced due to physical constraints). Then figure out DP, PD, and PA, because you're going to want them for some of the other calculations, like width and "is this going to break under normal conditions". The next thing you do (after running the aforementioned calculations) is to find your gear or gear stock (or a substitute, if that size isn't available). Hole patterns and CAD are near the end of the chain in terms of when you want to deal with an item--right around the light-weighting stuff.