Calculation is pretty empirical based on venturi, throttle, booster od and id, and a factor for the choke plate. There is also a factor for the "smoothness" of the entry, for example some of the modified Holley's with no air horns and everything super smooth. These last two "factors" are empirical and just set by looking at the shape, and then they are related to the other sizes (venturi and throttle). After tuning this little program on everything from the Stromberg 97 to the Holley Dominator, it is pretty good if I have a picture of the carb and the venturi, throttle, and booster measurements.
