I have no actual facts to back this up, but it would seem to me that to throw similar patterns from the various gauges the constriction should be proprotionate to bore diameter, IE the reduction of area be the same. Lookung at various factory specs for chokes over the years has revealed they generally & loosely somewhat follow that pattern.
Based on this constriction of a 28ga equivelent to a 12 ga would be as follows;
12ga------28ga
.040"----.030"
.030"----.023"
.020"----.015"
.010"----.0075"
.006"----.0045"
CYL-------Cyl
Note the constrictions for the 12ga are essentially the "OLD" card & felt/fiber standards for full, 3/4 (Imp Mod), ½ (Mod), ¼ (quarter choke), Imp Cyl & of course Cyl is 0.000 constriction though anything .003" or less is often referred to as Cyl. It is also noted that many US shotguns over the years stamped IC have traditionally carried ¼ choke constrictions. Many makers of course also varied some from these exact standards.
