The problem with the various charts is that different manufacturers--even in the same country (USA, for example) have used different constrictions as their standard for a given choke in a given gauge. And in some cases, those constrictions differ significantly. Jack O'Connor provides a chart of the standard constrictions used by Browning, Remington, and Winchester. 12ga modified, for example:
Browning: .024
Remington: .018
Winchester: .016
Those who've measured chokes on Belgian Brownings have usually found that they're almost always on the tight side based on what the choke marking tells you.
