as far as converting SCHRS to PHRF, I would just do the following. It's all just a fart in the sky anyway:

SCHRS:
F18 Rating: 1 (scratch boat)
Phantom Foiler Rating: 0.89
Hobie 16 Rating: 1.191 (for sanity check of math)

Portsmouth Ratings:
F18:62.4
H16:76.0
Phantom: X

1/62.4 = .89/X ~= 1.191/76

Hobie 16 rating value between the two is off only by 2% so the math sanity is roughly, approximately, right if I hold my thumb up high into the light.

X = (.89)/(1/62.4)
X = 55.5


I would scratch in the Phantom at a portsmouth of 55.5 based off SCHRS

For comparison, that puts it up there with an RC-30 / RC-27 in portsmouth...which I wouldn't think is enough of a difference by default. The Nacra 20 is at 59...IF he's foiling, they're going to blow that number out of the water but if the wind is light and it can't foil, it may have a speed potential less than an F18. In the vein of having some sort of justification for a starting point, I would run with 55.5