NOTE: This is by no means authoritative.
I used a simple set-up to derive the spring rate for a single, stock fork spring from a 2006 919. Now, this is a dual-wound spring, so it behaves like two springs in series until the softer spring fully collapses. That is to say, the spring equation is:
Ktotal = 1/(1/Ksofter + 1/Kharder)
I was not able to compress the spring far enough using my simple setup (drill press, digital weight scale) to collapse the softer coils, but I thought, maybe, the data could be useful to someone.
so, here is the data:
stroke (inches) | force (lbf)
0.00 | 000
0.25 | 10
0.50 | 19.5
0.75 | 30
1.00 | 40
1.25 | 49.5
1.50 | 60
1.75 | 70
2.00 | 82
2.25 | 92
The spring rate stays very consistent for the first two inches of stroke at 40 lbf/inch, or 0.714 kgf/mm. Of course, if you dial in max preload with all OEM parts, this takes up at least 25mm of the spring stroke. That gets you that much closer to collapsing the softer spring coils.
Like I said, it's not great, but I thought it might help someone. Setting up an assembled fork tube would allow me to test the spring through the entire stroke, but honestly, I don't care. New springs are practically mandatory regardless of the numbers.