It really all depends on the brand and the person who designs the choke tube. It can certainly be as William recommends, .675 for his chokes, or as the Browning engineers recommend .640 for their chokes.
That's what makes it interesting when we discuss what choke works best for what gun.
"Internal Geometry" and not the exit diameter makes the difference. Constriction (the difference in the nominal inside diameter of the bore and the exit diameter of the choke) is but one part of choke design and it's really all about what's going on inside the tube, angles, parallel, ports, etc.