OK, here's the process, Eric's right though:
1. Take out F1's.
2. Take out National F2's.
3. Count how many have done 4 rounds.
4. Work out 20% of the above number - that's the number of F2's (excluding FR1's and Nat F2's as above).
5. Work out 30% of the number you got in 3 - that's the number of F3's.
6. Work out 30% of the number you got in 3 - that's the number of F4's.
7. The rest, even those that have done 1 round, get F5.
BUT - you can only drop one grade a year, so if you were F2 this year, but ended up in the F4 bracket of the results, you'd be F3 next year.
Make sense?
G