How big of a vacuum leak would this be?

[packratbimmer]

The throttle plate should be completely closed, but one should know what that really means: I think the clearance between the throttle plate and the bore is .0015". The plate should never be able to contact the bore.

[noid]

The throttle plate should be completely closed, but one should know what that really means: I think the clearance between the throttle plate and the bore is .0015". The plate should never be able to contact the bore.

The below is the factory procedure for measurement and adjustment.

You're correct that you don't want the plate to bind with the bore; in effect the prescribed clearances is that of a human hair.

The intention is for the throttle lever to rest on the adjust screw at full rest, but only just.