The format is 5xxyy where xx equals the station number and yy equals the radio number. Radio numbers are not always uniform for every company (i.e. some stations use 05 on the chief's radio as a throwback to the old voice IDs used on the radio).
I suppose I should have tried to clarify what the original poster was looking for. The info I posted is the actual identifier that the radio displays on the trunked system (i.e. what shows up in Unitrunker, on a console, or on the screen of another radio, and on the screen of newer scanners).
Voice ID's are different and are based on plain speak terminology (i.e. Chief 53, Squad 53 as opposed to 53-5, 53-21, etc.)