I just discover that table made by pfaffi:
2 points are very interesting to note in order to be able to start and sustain the planing as of 7 knots:
1) for a light weight rider of 65 kg, a fin lenght of 53 cm should suffice according to his theory. That is a correct estimation because what I've found to be a good fin lenght by trial and error is that I have used in the past a 54 cm fin with a 10.6 m2 sail (a 48 cm fin was too small for low wind range).
2) Pfaffi used a Formula HWR board, a 70 cm fin and a Severne Reflex 12 m2 sail to be able to start and sustain the planing as of 7 knots. He weights 92 kg.
If his theory holds true and according to his table, you should be able to do the same with a 75 cm fin for your 100 kg body weight.
However, the important point is that your Fanatic mega cat cannot hold a 75 cm fin. I'm sure that the new Formula 167 Wide can hold a 75 cm fin.