i weigh about what you weigh and sail a MEQ2/Mistral Equipe II XR
with sails 7.0-10.0 and 8.x (8.4 race and 8.5 HSM SPF) being my favourite
when i repaired the gasket I never sliced it for the centre board i.e. also sail with NO dagger
just cover the top with tape to keep the water out
just recently i measured the OFO/one foot off from the tail and it is 38 cm
this corresponds with the fin that I received with the board
people are suggesting NOT to go longer than the OFO - tail walking, etc
however, not sure how that can happen with such long boards
i have used shorter fin / 34 cm when going with the 7 meter sail
have considered trying slightly more than the 38 cm fin, but nothing extreme - like 42 or so
let us know the OFO, test results, etc
so far the 38 cm fin has been more than adequate for me on the MEQ2
regardless of sail size
hard to spin out such a long board

and NO issues going upwind without the dagger - longer fin may help this
.
regarding the OFO "rule"- foils are sometimes one meter long in the mast - no more rules ...