Your not thinking way off at all. The problem is what rate? Right? Here is a way to figure out what you need. Regardless of what spring rate you now have, but you need to know what the spring rate and the free length is, jack up the car, reduce all the preload to "0" preload on both shocks. Let it down, move it forward and back some to let it settle. Make sure to add driver, passengers, cargo, etc. Now measure the length of the spring under load. Multiply the distance compressed by the spring rate and that will give you the static load per shock at ride height. Lets say it compressed 3" and you have 400lb. springs. Then there is 1200lbs. static load. Ideally you want about 1/2" to 1" of preload and the car to sit at approx. 25% with the short 5" shocks. That would be an additional 1.25". So 1200lbs. divided by 2.25 = 533.33 springs. Now the car will sit at the desired ride height with the normal load and the proper preload. This is only an example to show you how to determine what you need. FYI, with the AFCO's 2 seat rear springs are 300# to 325#, 4 seat/utility 350# usually to 400# for a guy that runs hard with 2 adults in the rear.