See if you can find this: 3/31/2017 Shotgun Sports | Pattern Analysis For Busy Sportsmen by Ron Jones (
https://shotgunsportsmagazine.com/archive/mar13/story0313.html) Cites Oberfell and Thompson,
The
Mysteries of Shotgun Patterns (Oklahoma State University Press, Stillwater 1957). Here is a table from the book:
Pattern Percentages for All Degrees of Choke for All Distances
Pattern Percentages Distance in Yards 15 20 25 30 35
40 45 50 55 60
Choke Full 100 100 100 96 82
70 58 48 39 32
Modified 100 100 96 83 71
60 49 39 32 26
IC 100 96 83 71 60
50 41 32 26 21
True Cylinder 98 83 71 59 49
40 33 26 21 17
Rough approximation is you lose 10% every 5 yards after 25 yards.
