To start, assemble every other leaf with a total of no more than 6.
You can check the spring rate easily with nothing more than a yard stick.
It will be trial and error on assembling the spring with various leaves.
Assume you weigh 200 lb. measure the height of the front end at a certain point.
Step up on the frame and measure the height again. If the car goes down 2 inches, you have a 100 lb per inch rate. This is a good starting rate. After the car is all done, check the handling and see if you want to go lighter or stiffer.
If you are not sure what "good handling" is, find an EXPIERENCED race car driver and let him advise you.
The final results will be, how many races you won all season.