For the Big Bar, Soft Spring asphalt setups, fine tuning is usually done with the RR spring. If you think you need a 400-pound spring in the RR, you can install a 375 spring and a 25-pound spring rubber. If the setup is off slightly in either direction, tight or loose, you can either add a stiffer or additional rubber, or remove the rubber to tune the setup. Then, when you're sure of the correct RR spring rate, just install that rate of spring for the race.
The advantage in using spring rubbers to change our spring rate on each corner of the car is the speed and ease of making those changes. We just need to be sure not to guess at the resulting rate change when we throw in or remove the rubbers. Use a spring tester and rate your spring rubbers for each corner and each spring that will be used in that corner. Then the change in rate will be predictable and we will be able to see the true results.