Due to a variety of stovebolt camshaft options that have been available over the past several decades, no hard and fast rules can be set in stone. Of course, the purists who bow down and worship the factory service manuals from 60+ years ago will disagree, since Moses brought those manuals down from Mt. Sinai millennia ago- - - -chiseled in stone!
The purpose of valve lash is to slowly take up the "clearance ramp"- - - -the short section of gradual lobe lift that stacks the cam lobe, valve lifter, pushrod, and rocker arm together before the valve actually starts to lift off the seat. That prevents noisy valvetrain operation and rapid wear from excessive clearance, and also stops valve burning from the valve failing to close all the way at the end of the cam lobe. The only way to be absolutely sure of correct lash adjustment would be to check every cam lobe with a degree wheel and a dial indicator and adjust the clearance according to the lobe profile. That .008/.012 is a pretty good starting point. At least it assures that valves won't burn from being too tight. If they're noisy at normal operating temperature, sneak up on quiet operation a little at a time. The big thing to remember is that there MUST be a little slack on all the rockers with the engine hot.
Now- - - -hydraulic lifters open up a whole different can of worms!
Jerry
