Best way is to do the math,
or try an online calculator.
What revs do you want for what speed?
If your going to race the bike, I'd be getting a couple of sprockets for different tracks.
normally your looking at getting max torque at 1/2 max speed for a tight track,
but only testing will give you your ideal sprocket size.
Maths will get you un the ball park though.
Start with max revs, and desired top speed, work out wheel Rpm at top speed, then work out the ratio of your jack shaft and you will have desired final drive ratio.