Some details on my rear spring/damper set-up.
The rear springs on my Mk2 have an uncompressed length of 254mm (10").
As can be seen from the photo, the compressed length of the main spring plus the helper spring is just over 9" - about 1/2" of that is the helper, so the compressed main spring length would be about 8 1/2". The photos are taken with the car on its wheels in normal parked position.
On full droop, the helper spring is almost fully uncompressed at just under two inches, so suspension travel from static compressed to full droop is about 3 1/2". (1 1/2" main plus 2" helper.
I have the spring seats at their lowest setting. Without the helper spring they have to be wound up over 2" to keep the main spring located, which obviously raises the rear ride height by that amount. I did consider having some springs made with a greater range, but I've been happy with the way the car handles with these springs so opted to fit the helper springs. They are from Faulkner springs, and take 4lbs to compress. Dampers are AVO.