Reading back through the thread, I think the confusion came from:
Stuart wrote:So if you have 4" of travel say, your 75 lb springs will be bottomed out by a 600 lb load or 272 kgs, surely the rear of a 800 kg ish car weighs more than that?
If your springs are 12" to start with then I would imagine that you have at least 8" of spring travel before the spring itself is bottoming out, or becoming coil-bound as it is known. Thus the actual travel limit is set by the damper, which is probably 6" in total.
If you have set the springs with the damper fully extended and 12" between the spring seats, then the helper spring will be virtually closed and the main spring will have a small preload. The static weight will then take up 4" of travel, leaving 2" more for absorbing bumps.
However, if you have fully compressed your 2" of 60lb/in helper spring, then that would be a pre-load of 120lb which means that main spring will also be compressed by 1.6 inches (1.6x75 = 120), thus the static load will compress it a further 3.4 inches, leaving you with 2.6 inches of travel.
Preloading the spring further will increase your ride height and give you more suspension travel, until you reach the point where the pre-load is higher than the static load. From that point on you have solid suspension until the dynamic load and static load combined exceed the pre-load. Could you have already exceeded that point? In which case that might account for the initial harshness that you mention.