3/16" according to the '64 shop manual for stock carb. Looked closer at the book, and there's two count 'em, TWO choke adjustments for these manual carbs!

One is the fast idle adjustment. They show the carb removed and held upside down to make this one, with a drill or gauge (size not specified in the manual) between the butterflies and the fast idle screw just cinched up. Removal isn't really necessary as it's just a bench "ballpark" setting anyway. Just set the fast idle screw for the desired RPM with the choke pulled out. 1900 is one listing on a data sheet.

The pic I posted earlier out of the manual is for the choke pull down, a different adjustment. I never paid much attention to it. Not surprisingly it was way out! What's supposed to happen, when the choke is "fully closed", it really isn't, the idea I think is that the 3/16" clearance (made by adjusting the nut at the end of the pulloff rod) the choke plate will then also have a little spring action or "flapper valve" feature. It will probably even open a bit more too during actual cranking as air is drawn through the carb. The small nut is friction fit (not round) had to back it off a bit to get it adjusted, but it isn't going anywhere. A drop of blue or green Loctite would do the trick too. Once set, you can forget it.

The one thing have noticed about these carbs is even small adjustments, in every setting float height or idle mixture etc, makes a BIG difference in how it runs. They don't really need any attention after they are setup, but take the time to do it right the first time.

Another thing when starting in cold weather, the choke doesn't just reduce the amount of air flowing through, though it does that, it also increases the venturi effect so that more raw fuel is "pulled" through to aid in starting. If the choke plate were completely closed, it may not do this effectively.