Start around 38mm/rev, and then see what the car is like to drive and how hot the motor is. Use the gear charts available above to select the gears you need.
If the car leaps out of the corner, and the motor gets hot, drop down to about 36 and try again. If it spins the wheels up and seems slow on the straight, go to 40. You can go on from either of those to find the ratio you need.
If you have a speedo with timing-on-the-fly, start using it only when you've sorted the car with no timing. HTH
