Of course the only way to determine what any barrel patterns with any given load is to "Pattern" it. A gun throwing a given pattern percentage with todays shells will normally have less constriction than it used to.
All that being said to answer what I believe is the intent of the question choke is generally proportionate to the reduction of the "Area" of the bore.
Thusly a 12ga bore of .729" with a 10% reduction of area would have a choke dia of .6915" or.0375" constriction.
Likewise a 20ga of .615" bore with a 10% reduction of area would have a choke dia of .5835" or .0315" Constriction.
These Should be expected to have a similar spread, ie % in the 30" circle. The equivelent reduction of .010" in 12ga would be acheived with about .0085" in a 20ga. In this line note that a reduction of .010" in the 12ga was truly a 1/4 choke not IC though often so marked. A true IC in 12ga was generally only around .006" constriction & even less in the 20.
