Playing Card Standard
While the playing card standard is an authentic way of judging accuracy, it ignores the time factor (and of course the stress factor of someone shooting back.)
I concur with the accuracy standard of six shots into a playing card at 7 to 10 yards being excellent.
I think 2 shots into the center scoring area or head of an IDPA or IPSC target at the same distance in as many seconds is a good "gunslinger" demonstration. With range safety considerations, doing that from a holster draw may not be practical.
However, never lose sight that the standard for Hikok, Earp, Matterson, and others as "man killers" was to walk away.
I offer that while they all had to be good marksmen, their willingness to face death was more important to their survival.