SCHRS publishes a conversion to RYA Portsmouth: https://www.schrs.com/pylookalike.php

Basically their published value for a Nacra 17 C-board is 67.2. For an F18, its 67.8.

Hence the delta is 0.6. Going over to US DPN, (table here: https://www.ussailing.org/wp-content/uploads/2018/01/2017-Portsmouth-Precalculated-Classes.pdf), the F18 is 62.4. Subtracting 0.6 from this gets your Nacra 17 C-board boat a number of 61.8. That seems reasonable. The other approach is to use the SCHRS number more directly, i.e the Nacra 17 C-board is rated at 0.991; the F18 is 1. So 0.991*62.4 = 61.8.

That is probably the most reasonably accurate number you are going to get, and it does make a bit of logical sense. The Nacra 17 C-board boat is effectively a Nacra F18 Infusion with C-boards, carbon rig and ~70 lbs less weight (146.6 kg vs. the 180kg F18). That would mean more if they didn't cut the sail area down some. All in all a good boat with t-foil winglets but not a ton faster than a stock F18.