[MrForbes]

It's pretty easy to get close numbers....find the no load rpm of the motor, find the wheel diameter, decide if you want a wheel on each side of the ball or just one side, and figure out the gear ratio needed to make the wheel(s) spin that fast, or just a bit faster.

If the motor speed drops too much when shooting the ball, add another motor, or add mass to the wheel and wait longer for it to spin up