The clearance you are talking about is the valve clearance..(between the base circle of the camlobe and the bucket/lifter). On stock 300zx lifter (hydraulic type), the clerance is 0" as it will self-adjust. I am using Tomei solid lifter which require 15 thou on the exhaust and 17 thou on the intake per Tomei spec. I asked this question too a few days ago :)
The clerance I am talking about is the clearance of the valve spring at max lift of the camshaft. Basically, it's how much more you can compress the spring before it will bind (become solid) when the valve is at it's max lift. Hope this help. I'm trying to figure out wether 30 thou clearance as measured is enough compare to the recommended 50 thou minimum rule of thumb.