Ian,
I had more than a coils worth of skirt remaining. Perhaps it wasn't quite 30mm. But I remember that I had to drop the front of the trailing arm to get sufficient movement on the lower wishbone to get the 7" springs in so now, even at full downward travel of the rear suspension, the coils are under slight compression.
Interesting comment about there being no witness marks on the skirts. Mine were the same. Probably the only bit of decent powder coating on the whole car.
Nigel