I've never seen a true "specified" clearance. Ancient Yugoslavian mystery..
Easiest thing I would think
, is take a sample of 5 to 20 functional rifles, get an overall average i.d. on a working gas block and the o.d. on the gas valve. Then measure a new U.S. made gas valve, and a old sloppy leaking gas valve, this might give you how much it takes to exceed the leakage rate.
Call it redneck sasquatch ballpark clearance.