Ok, so after reading all those threads and links on carbs over on dot org (thanks, Dan), I see that the correct info on the vent clearance for what year carb you have is not given.
It's not a terribly crucial adjustment. As long as the bowl vent operates correctly (opens at curb idle, closes above curb idle) its exact degree of opening has only very minor effect on how the engine runs.
Is this possibly why there have been so many idling problems for the Holley 1920?