You're right about the size of the teeth having no bearing on the amount of reduction. Maybe I didn't word my explanation clearly. The drive sprocket to the rear wheel needed to be at least 2.5" in diameter for the chain to not rub on the swingarm (I said 2" in the last post, but going back over my notes it's 2.5") . If I was using a #35 chain for the drive, I'd need a minimum of 20 teeth to get the 2" diameter. Using a #41 sprocket, I could get away with 15 teeth.
Although there were rear sprockets available with a higher tooth count in #35 chain, a big enough one to give me the reduction that I wanted would be a special order sprocket and it would be too big to fit in the swingarm. The #41 setup worked the best.
The bike is all done now and it's a blast to ride. The gearing worked out perfectly. Any taller and it would stress the clutch, any shorter and it would be too slow. I don't have a way to put any videos up, which is too bad because I had a spectacular crash on it while I family and I were having minibike motocross races with it. I came across the finish line at full speed and tried to do a dramatic rear slide to a stop, but I ended up highsiding and flying off, shirtless, into a pile of dog poo. My wife and daughter were very amused.
When I get my camera back I'll post up some pictures, though. I couldn't be happier with how it turned out. With the motocross chassis and suspension it can actually be ridden pretty aggressively.
The desire for safety stands against every great and noble enterprise. -Tacitus