So this is my first time installing coilovers I've watched a few videos on YouTube did some google search but I couldn't find much about the spring bucket n adjusting it. My question is..... do I adjust the height of the rear by adjusting the spring bucket or from the actual shock.. it's a 3way adjusting coilover..

 

3-way adjustment refers to damping, not height. There is a piece that sits in the bucket that provides height adjust for the rear. Unless this is a true-type, which I don't believe it is.

http://www.the370z.com/brakes-suspen...ml#post3338597

Check out the Powertrix set in post 17. The Tein will look very similar. Note the rear height adjuster is at the top. Tein may be this way too, couldn't find a photo of Z/G set and made an assumption.

 

Height is adjusted by the spring bucket. You'll adjust the damper as well so the preload remains the same. Zero to 10mm is ballpark. Measure at the lower shock mount eyelet centerline.