Springs are wound with the same gage wire, so the stiffness is defined by number of coils engaged. If you look at the pictures in this thread
http://mirageforum.com/forum/showthr...-spring-thread there's roughly 3.5 coils in compressed state, and another 6 locked out. So the rate should be 3.5/9.5 or ~1/3 on rebound. The wind in middle section is pretty uniform so it is pretty much on/off no transition. For the part of rebound stroke middle section stays locked.
With vinyl tubing gap between middle coils was ~1/4", and there is ~390lbs of load per rear wheel. Compressing first stage gives 1/4"*9 = 2.25" * 50lbs =113lbs, then 265lbs/150 = ~1.75" of 2nd stage.
Interestingly without vinyl tubing we'd have 3.4" 170lbs of 1st stage 1.4" of 2nd stage for 0.8" loser ride height.
This is rough estimate too lazy to do it correctly.