Before reading that article I was guessing 200-300 lbs and it appears to back it up.
I def agree with Bob though on seeing where you are and going from there...
Also found this:
How do you measure spring rates: Grassroots Motorsports forum: Grassroots Motorsports Magazine
"HOW TO RATE COIL SPRINGS
Spring Rate = GD^4/8ND^3
G=Torsional Modulus for Steel = 11.25x10^6
D=Wire Diameter in Inches
N=Number of Active Coils
D=Mean Coil Diameter in Inches
8=A Constant for all Coil Springs
Copied from another forum. Didn't 'fact check' this."