It's been a long since I did this but I had the same problem. The US Sailing conversion is not applicable to multi's. If you try to sail with that rating and win, you will be protested and thrown out. There may be something about this in the Old Forum

I ended up using a method developed by the Corsair's. In simple terms it's hard to convert directly from PHRF to Portmouth because they work diffrently, PHRF is handicap based on distance sailed and Portmouth is handicap based on time sailed. However PHRF has PHRF(TOT) which works just like portmouth so there should be a simple relationship between them. (see http://www.nemasail.org/lussier.html). For data we used a boat with an accepted Portsmouth and PHRF, the F27 with a PHRF of 60 and a porstmouth of .72. I stiil have the spreadsheet which gives


INPUT-> 60 PHRF(TOD)
calculated 1.07 PHRF (TOT)
calculated 0.94 1/PHRF (TOT)


OUTPUT-> 0.719999646 Test Calculate Portsmouth# should be .72



INPUT-> 0.626 Portmouth #
OUTPUT-> -19.64 PHRF


I will look around and see if I can find the details.