What WE did to "correct" as much as possible the variance between PHRF and Portsmouth and still be able to use the PHRF handicap of distance on time, was to rate all boats first in Portsmouth (that means all the monohulls and cruisers). Then we converted all of the Portsmouth numbers to PHRF. This was the only way to have a somewhat fair reverse handicap race using a time on distance handicap, as Portsmouth is time on time.
Regarding accuracy, to put too much faith in any of the systems is a big mistake. PHRF has always been a dart board system with a heavy political mix of opinions of what a boat "should" rate. Hence the differences in various areas.
Portsmouth is only a little better as it did at one time actually crunch some numbers based on races. That said, you were still not taking into account the sailor's ability.

As an example, lets say a new boat comes out and races and Randy Smyth is sailing the boat. Another similar boat comes out and I sail that one. Then we come up with a handicap for the 2 boats. Guess which boat gets a golden rating and which one has a rating that "no one" can sail to? Picture that across the country. Not to mention the folks who push and whine for a better rating, if they are the "right" people, bingo they get an adjustment. etc.

Bottom line is all of us are desperate for a system that could rate all boats and offer a time on time or time on distance option for Club's and organizations to include "everyone" in their races. Let us know how your system develops.

All this said, remember that handicap racing is intended to be just for fun. None of it was ever consider to be really accurate. It is for Club racing and "ball park" comparisons.

Cheers,