the spring rate is whatever is needed to just hold the car up
2 things are missing:
1. there's more than one way to get the car to the same ride height (assuming you know what you want already):
A. short stiff spring - stops transferring weight when it tops out
B. long weak spring, the extra length is pre-load - transfers weight up to its full length
2. spring stiffness must be enough to control the weight of the unsprung components (axle etc.). An 8-1/4" and a Dana need different rear springs regardless of the rest of the tuneup.
The spring has to be the correct rate to hold the car up at the desired ride height when compressed to the desired length. The spring rate and the resulting compressed length is a tuning decision. Having the spring in coil bind before the shock is fully compressed OR having a spring that holds the car up with zero spring compression are the only wrong choices. The choices between those 2 extremes are determined by how you want or need the spring to influence suspension movement. If a spring "tops out" or reaches free length at any time during a run, something is wrong.
A lighter spring has to be compressed more which stores more energy than a heavy spring which is only slightly compressed. This energy can be used to help get the housing out in a car that needs help getting the tire applied at the hit. Higher horsepower applications don't need that influence and will use a heavier spring with less energy stored at ride height. Think about the 6 cylinder torsion bars helping get the front end separated on cars that need the pitch rotation. Same effect.
While the weight of the unsprung components may influence a spring choice (total weight-unsprung weight=sprung weight), the spring doesn't control what the axle does. It influences it, but the shock is what controls it. Suspension type and geometry, spring rates and shock adjustments all work together to determine what happens as the car moves down the track.
