You probably already have the best fin!
Last season I used 44cm most of the time on my 117w with 8.6 - 7.8 - 7.0. I was using a Z-Fin 44cm which should be pretty similar to the Boss. For extra lightwind performance with the 8.6 you can go for a 46cm but I found I couldn't go any smaller than 44cm even on the smaller sails. I was 80kg last year riding the 117w.
GetWindsurfFit.com // The Ultimate Windsurfing Fitness Program.