I posted the almost exact question in the rookie section earlier ;-)...

spur and pinion size question
With regards to the FDR question the equation would look like this:

SPUR / PINION * INTERNAL RATIO = FINAL DRIVE RATIO (FDR)

F.EX. my T2'008 has an internal ratio of 1.7 so the equation with a 84tooth spur and a 34 tooth pinion would be:

84 / 34 * 1.7 = 4.199

You could of course work backwards to figure out your ratio between spur and pinion.

Say your max FDR is 3.500, like is it for me, you would get an equation like this:

SPUR / PINION * 1.7 = 3.500

some math....

SPUR / PINION = 3.500 / 1.7

more math...

The ratio between spur and pinion would be:

SPUR / PINION = 2.058

Meaning that your spur should be 2.058 times bigger than your pinion...

.............

As for the rest of your question I'm my self at a loss ;-)