Roger, you might want to get John's definition. But, remembering that I am not a seismologist, here is what I would do: for a M8+, anything within 5 deg of latitude distance (meaning combination of lat and long) is an aftershock. For 7.5-7.9, anything within 4 deg, for 7.0-7.4, anything within 3 deg, for 6.0-7, anything within 2 deg, for 5.0-5.9, amything within 1 deg. Or, if that is too hard to program, just anything within 5 deg. These distances are a little longer than the rupture zones and would include some close-in quakes triggered by static stress changes-hmm-would Big Bear be an aftershock? I think so.
Chris