Basically, you should adjust the height using the bottom mounts of the coilovers, NEVER by using the bottom spring perch. That should be tightened just enough to hold the spring in there and not allow any movement. There will be a locking collar that's screwed against the bottom mount, so loosen that and turn the mount one way or the other to raise or lower it. Also make sure that only the bottom mount is turning when you're adjusting it.
I'd recommend counting how many turns you adjust each one so you can change everything evenly left/right and front/rear (if the car is level as it is).