Portsmouth handicap calculations aren't very difficult. There are several worksheets floating around that lead you through the steps and include handy charts for converting seconds to decimal minutes. The hardest part is dividing the decimal elapsed time by the handicap number - and if you have a pocket calculator that's no problem. I've helped run several regattas (PHRF and Portsmouth) where we've calculated the scores manually on the committee boat while motoring back to shore.

Regards,
Eric