I used to have a 2012 Falcon 99, 62 wide, and found a 36cm Meanline worked very well with the 7.3m. Given the boards are a similar width I would have suggested around 36cm for you. For reference, I am 90kg.
The Vector Australia website has a guideline that doesnt match up exactly with your dimensions (width/volume), but whether you look at volume or width the answer generally comes out at 36cm as well when using a 7.5 (for all of their slalom fins : Volt, Rockit or Canefire).
www.vectorfins.com.au/compare