We just took the easy option and halved (i think?) any vis that a civvy airfield gave in a taf/metar unless they specifically reported something else. Think that's the criteria for prevailing vs specific vis? If prevailing vis is no more than double the vis in any direction its all that's reported unless blah blah blah some rule about low vis, can't remember exactly at the moment.
In short, believe nothing a met man tells you!