Fwiw, for the comping we aim at 3G of spring rate over the travel at the rear - i.e, if one rear corner has 500lb of weight on it and 10" of travel, I'd aim for a wheel rate of about 150lb/inch, if the coilovers hit ~8" down the arm then about 240lb/inch at the spring depending on the angle of the coilovers.
Depends on what you're doing really though, how much travel you have, whether you want a progressive setup, what the rear unsprung weight is, damping.......
I tend to go the opposite way to a lot over here and the more travel that becomes available the stiffer I'll go on the main spring - but make it shorter and run a longer, softer tender and let damping and extra droop travel do the work over the washboard stuff.
