My ongoing personal calc revision testing indicates that a 4500lb vehicle with 50/50 weight dist. would be good with 950lb springs, and bump/rebound values of 5/9
I have yet to play with any cars in FM3 heavier than 3400lbs though, so this is just based on extrapolated theory at this point.
I begin indepth weight/spring testing tonight, so I won't even be able to confirm my own thoughts on this until probably tomorrow or saturday.
EDIT: A 3200lb car should be good with about 700lb springs.
Note: I've been altering my damper calcs too, so my springs may seem a little low.