Depends on the conditions. In our fleet we race with schrs rating, the Tornado is the king with 3-4 bft, A cats win with less wind and maybe the H16 might be a candidate for > 5 bft (no F18 here, so I don't know).
Anyhow, on small races with diferent boats the final result is your time multiplied by some theoretical factor and this makes the result more than questionable. It's a way of getting mixed fleets together, but don't take the results too serious.