Had found some information on spring rates,
Seems the aftermarket springs makers have to make a certificate with specs.
Germany's TUV certificate,
For example for H&R it is on their website under service/product
i.e. the 28802 product for M2 document,
http://www.h-r.com/bin/28802.pdf
From there and with the calculator you can get a ballpark estimate of drop and rate (assuming springs compresses linearly)
Also, let's not forget influence on the spring rate of the rubber secondary spring..ie on a 235i seems 8mm of travel will start engaging rubber bumpstop.