What are your intended uses for the car
The actual spring rates don't matter(to an extent) it's the frequency that matters. Even the wheel rate doesn't matter, you just need that to plug in to the formula to get the frequency
By my calculations, 1,000 rear spring rate gives you 1.99 HZ, that's with the rear weight being 782#'s unsprung
A front rate of 310# should give you about 1.78 HZ. 400# is at around 2.14 HZ
A rear rate of 725# gives you about 1.79 HZ.
These are off of the weights being 889 lbs front, 782 lbs rear (sprung mass)
You generally want a rear frequency that's 10% (or maybe its 15) higher than the front for a street car, to make the car not feel like its pitching back and forth.
http://www.e90post.com/forums/showth...t=85206&page=2
http://eibach.com/eibach/img/ers-14s...nworksheet.pdf
Take the number you get from the formula on eibach's site, and divide it by 60 to get the frequency.
It takes maybe 20 minutes to read this thread, plug your #'s into the formula on eibach's website, and calculate what spring rates are acceptable for your needs.