A good starting point would be to use springs equal to your corner weights. This is not scientific but will give you a good starting point. The car won't oversteer on you and you can get some comfortable seat time. I would expect once you can predict the car's nature, you will be going up on the spring rates in the rear to help induce some oversteer. You can also put tie raps around the shocks as a poor man's weigh of measuring how much your suspension is compressing.