As guns built for old card & filler wadding in 12ga were considered full choke with about .040" constriction (bore reduction of 11%) & guns for new shot protected shell are considered full with often no more than .030" constriction (8% bore reduction) it would seem to maintain a proportionate ratio in the 28 ga .030" would be ample for old shells & no more than about .025" for new ones. "Personally" (I am aware all will not agree) if a 28 bored about 5/15 proves insuffcient for the task at hand I would just reach for a larger gauge, rather than more choke.
