As Stan stated the true choke is determined by flattering. As for bore constrictions, this should relate to a percentage of restriction, rather than a certain number of thousandths of restriction. Thus under old standards for card & filler wads, a full choke had about a 10% reduction. This amounted to around .040" for a full choke 12, but only around 032" for a full choke 20. Note these percentages have t be worked using the Square of the bore diameter, not the actual size.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra