rear engined suspension tuning (spring rates etc)

[juse]

To Tim;
How did you end up with figures like 200/250 lbs on springloads?
I`ve been trying to study these things for a while, and the loadratings seem to variate between 80-150 lbs.
Someone wrote that 80lbs is common with aftermarket struts in U.S., one guy who drives autocross uses 100lbs, one ended on 120lbs and Steve Carter uses 150lbs.
Is it just a matter of opinion or why the scale is so wide?
Yesterday I heard from a Finnish guy who drives rallysprint that he`s not satisfied with the springs he is using now (160lbs), and he tought that he would try 200-220lbs.
Justin