You may have covered this, there are 2 other dimension worth considering:
A) the distance from the eyelet (mounting point) to the platform of the spring collar. The shorter this distance the lower the ride height for a given spring.
B) The length of the body of the damper. If you use the front Spax damper from a TC Europa on a S1/S2 Europa with stock or shorter springs the damper will bottom out as the damper body on the TC damper is longer than that for the S1/S2.