It's a good idea to buy or borrow a good quality spring rating device so you can accurately determine the installed spring rate of each spring you might use. Compress the spring to the installed height and then rate the spring from there. Here we see a team measuring spring rate with the gas pressure shock installed to learn the combined spring rate that includes the coil and the shock spring rate caused by the pressure. Theoretically, we should rate a spring based on which corner of the car it will be used on because of the different ranges of motion. For the LF spring we would measure the height of the installed spring and compress the spring in the spring rating device to that exact height before rating. Then, for a conventional setup, this corner might bump and rebound up to an inch or more. Check the rate up and down 2 inches from the installed height. For softer spring setups, this corner might travel up to 3 inches or more in compression.