Keep in mind that our spring rate calculator provides a starting point...rates can be tuned to satisfy a particular car, driver, tires, swaybar, shock set up.
We worked very closely with Hyperco to develop this tool. They've been building springs for awhile. We took their lengthy and complicated engineering algorithms and translated them into a tool that most of us can understand. We've found it works pretty darn well. If we get a car that is on the extreme fringe for weight or traction sometimes it can miss a little, but such cars are typically serious track units that are subjected to an appropriate amount of tuning anyway.
The other cool resource we've developed is the huge library of vehicle weights that we've gathered over the last few years.
http://www.ridetech.com/tech/coiltech-2/
If you take a look at that library I'm sure you will find a combination that is darn close to yours. This will save you the trouble of finding a 4 corner scales for your own car. Then all you need is a tape measure and the discipline to use it!