In the "old" days , the amount of choke ; IM , Mod , IC etc ; were defined by the % of shot that would be in a particular circle at a particular distance . Truth be known , the amount of constriction that provided a particular performance may have been different from gauge to gauge . Another train of thought was that it was more related to "area" than to "diameter" at the muzzle .
Regardless , the only real way to determine how a particular choke tube/barrel or fixed barrel performs is to put it on paper . It is amazing as to how some combination is reported by the shooter who has never been near a patterning plate/frame , much less counted the hundreds of holes involved .
Charlie