The best way to do this is not to. A primary key should be completely indepedent of any attribute a record might have - not to mention other records (consider what would happen if a client is renamed, or deleted).
You can easily append the first letter of client's name to an auto-entered serial number (running consecutively for ALL clients). It's also posible to calculate dynamically each client's ordinal number within their "group". But neither of these should be used as the primary key.