I may not be using the correct rate myself, so I don't feel qualified to guide you. What I did do was:
Calculate the sprung weight at each corner. In this example I'm using 300 lbs for the front driver's side wheel.
Then I measured the angle from vertical of the shock at the static ride height (in my example, it will be 20º from vertical.
Then I measure two lengths on the lower control arm: the distance of the inner pivot center to the lower spring perch center, and the total distance between the inner pivot center to the outer pivot (center of ball joint). These two distances provide a ratio that will be used to provide a shock movement ratio. For example, if the lower shock is mounted exactly half way along the lower arm between the inner and outer pivot, then this ratio is 2:1 (shock compresses .5" for every 1" the wheel moves up), which when squared, means that whatever spring rate I choose (see further down the explaination) in an assumed 1:1 movement ratio, must be the square of this actual (2:1) ratio, or 4...4 times the 1:1 ratio (I'll tie all of this together in a minute). The angle of the shock now must also be factored into the equation. The sine of 20º is roughly .94. Which means the shock will move .94" for every 1" the wheel moves (as a function of the angle only). This must also be squared, so that now my spring rate (inverse of .88 is 1.13, or 13 percent) must also be multiplied by 113% on top of the 400% percent earlier, or now I must have a spring rate that is 452% larger than an ideal 1:1 movement ratio.
So, I now have a movement ratio (considering shock location and angle) that can be used to modify the next chosen value:
Remember the 300 lb corner weight from before? In my particular situation, I would like the wheel to move no more than 3" from full droop (spring has no pressure, but is not loose in the perches either) to static ride height, which compresses the shock to just under half way. This means that assuming a 1:1 movement ratio, the spring would need to be 300 lbs / 3", or a 100 lb/inch spring. Since my movement ratio is NOT 1:1, but instead a combination of factors that resulted in a 452% increase, my spring rate must now be 452 lbs/inch springs (100 lb/inch * 452%). This sounds high, but it's because of the mounting geometry I used in this example.
In reality, my project's front suspension movement ratio, squared results in a spring rate that is 190% of a 1:1 ratio (because my shock is mounted just inside the lower ball joint (almost touches it)), which mean I would only use a spring that is about 200 lbs/in. BUT, the 3" compression from droop to static is a little soft for me, so I'm stiffening that up to a little over 2", which equates to a front spring rate of about 275, which is what I have on the car right now. Will it be too stiff or two weak? I won't know until I drive it, but these same calculation were used on another car I built, and they were pretty close.
The rear suspension will have about 50% more weight on the springs, but the angle of the shock is more upright, and the shock is attached even closer to the outer pivot center, which means a lot better or higher movement ratio, so I can use a 300 lb spring in the rear even though the weight is so much more (better geometry for supporting the weight of the car).
Long story short, movement ratio (which can be measured more easily on the car than all the calculations I've done) squared is the value used as a multiplier for a 1:1 ratio. Your personal preference for soft or hard suspension, how much stroke your shock has, and sprung corner weight will determine what that 1:1 rate will be before you apply the multiplier for the movement ratio.
Hope that helps.
Ox
