Setting up coilovers?

[cebul]

That thread states 5mm is the factory setting though?

Yes mate, that is a fctory seting.

To change ride height is it just a case of moving the bottom ring C?

Yes , you need to make that ring loose, and then turn whole absorber - you can grab a spring to turn whole thing - that is how I'm doing, and when you lower it as you want to, just secure the ring C and job done