For good quality stock motors using 48 pitch gears a 96 tooth spur with pinions ranging from 23 to 27 teeth. Lower torque motors will stay in the 23 to 25 range. I always start with a 25. A rule of thumb I use for track testing is:
Primarily technical infield with long straight - Make sure you reach top speed at mid point of straight. Gives more torque to make up time during most of the track which is technical.
Primarily fast flowing infield with long straight - Make sure you reach top speed at 2/3 point of straight. Gives more top end to pull a few more inches before slowing for the next corner.
Mixed track - Measure yourself by the fastest guys out there and see if your faster overall even though they may be faster in the straight or coming off the corners. If you can stay on their bumper, or ahead of them, then stick with what you have. If not, go 1 tooth either way and see if you notice a difference compared to the others again.
I know you probably already knew this, but I always write my replies with others in mind like myself who are trying to learn and actually "understand" whats going on, instead of just going with whatever someone, official or not, says is right. Your not learning then, but rather following tradition.
just trying to help