do you still have the quicktime? it would be nice to start getting a bit of a database together. Calculations for the secondary is complicated for reflective values but is a good design feature. They make the combo less peaky and have a broader range.
think of them as having two spring rates together and hanging a given weight from them, it makes them progressive with a broader dampening range but you won't have that same peak loading point....maybe I just butchered the explanation
Gas dynamics and springs; probably not the best comparison.