So here are the spring rates I know people have used:
800/1100: user reports rear could be slightly softer
550/900: user seems to be happy but needs more testing
500/800: the kw rate which many have tried and seems to be a good balance
500/750: slightly understeer but a good, safe set up
Again, there is no magical formula. The set up could vary based on the track you are driving on, damper settings, and driver preference.
E92 M3 DCT