I'm running a 46 tooth spur on the slipperential, then I'm going to be running a shaft with a 10 tooth pinion and a 15 tooth pinion on it for gear reduction, so effectively my "spur" is going to be 15 teeth. The space is measured out to accommodate a 9t pinion up to a 17t pinion, and if I need to go higher than that, I can also always change the spur to be lower as well.

Also, I have made some progress on the drawings and will post up another version tonight/tomorrow. I think I've done it right this time.