I have a Holley 750cfm vacuum secondary with electric choke. With a cold engine, the choke is completely closed. No chance for any air to get in there. Shouldn't it be open just enough to get a drill bit in there? If so, how do I adjust that - by loosening the 3 screws and rotating the electric choke or by bending the choke lever?

Loosen the three screws and rotate the housing. Anyway the choke should be closed when cold, when the engine fires the airflow will counter against the spring tension and allow some airflow. Just experiment with it until you get the right setting.

rotate it until the plate is lightly closed @70 deg for a ballpark setting (& go from there) and yes the choke pulloff will open it slightly (from closed) from vacuum acting on the piston inside when it starts and there's a spec for that (use a drill bit to measure the gap between the airhorn and the upper edge of the plate). Not sure where/how you adj that on a holley.

Consider that on factory electric chokes (GM anyway) the choke power ran through an oil pressure sensor, to be sure the choke didn't pull off before the engine was started. Don't leave the key on unless you're cranking to start.