Hi,
PHRF is a time-on-distance rating. (sec/mile). It varies a little depending on region because of varying wind and sea conditions and the courses run there. I am on the council for my region, which I believe is extremely factual with no politics (we meet once a month with much data going back and forth).
Many(if not most) PHRF fleets convert this into time-on-time in various ways. It seems the most common is:
TCF=550/(PHRF+650).
This is the Time Correction Formula and is multiplied directly by the elapsed time to get the corrected time. Others use different values for the denominator - see http://www.ussailing.org/phrf/TOT.htm
which is an excellent article by John Collins.
Be careful about comapring Portsmouth yarstick from mono's to cats - this does not work. The 100 for the Thistle should not be compared to the 92.4 for the Hobie Wave, for example.
To convert mutlihull D-PN to PHRF is tricky. I've tried several formulations. Here is one I made up:

PHRF = [(650 x DPN)/75] - 550

This makes a Hobie 16 PHRF = 110 sec/mile and a "Classic" Olympic Tornado PHRF = 3. That means the Tornado owes the Hobie 107 sec/mile or, in PHRF time-on-time, about 10 minutes an hour.

According to Portsmouth Yardstick, with the Hobie at 76.1 and the Tornado at 63.8, that's about 11.5 minutes an hour.

Seems to work OK in that regard and also very crudely against the monohulls. (though it shouldn't be used that way - but it's nice to have some point of reference). I handicap for a race that has 120 mono's and sometimes 15 cats, and this seems to work OK, though we only score boats in their own division, it's still nice to compare.

Steve