You have opened a can of worms, and most everyone has their own opinions about what degree of choke does what on the pattern board. Understand you're entering an area of study that suggests "imprecision"....absolutes do not exist here.
A number of companies have their versions of truth - Google is yer friend.
Several problems arise when checking printed references. Books from pre-c.1970, for the most part do not take into account the change in patterns when using the (then new) one piece shotcup/wad. The printed or computer reference likely will not tell you the complete information on the particulars of the test shotgun and the test shot load make-up. Different size shot pellets apparently may change the "degree of choke" as interpreted fom the pattern board. Hard pellets apparently may pattern differently than soft pellets. Gross changes in shot charge velocity may affect pattern percentages.
Regardless of what information you find and how you think it applies to your particular shotgun, the proof of the pudding is the patterns it fires with YOUR ammo - and judging the patterns correctly. There are no shortcuts here, although the measurements of your barrel compared to the "charts" should give you a general guide, which is better than no guide at all.
Older shotguns with fixed chokes, will usually give notice of the degree choke either with the printed word visible on the barrel (eg. FULL) or by choke code symbols beside or under the barrel chamber(s), these codes or symbols vary depending on the manufacturer.
I suggest -
http://www.choketube.com/patterning-shotgun.phpTo answer your last sentence - I am away from my library for several months, so I'm running on memory. Several books will answer your questions, but the best IMHO is Shotgun Technicana - by David Trevellion (aka Crossed Chissels) and (the late) Michael McIntosh.
Hope this helps....my $0.02 and you're welcome to take it FWIW.
