First let me say that I have a Tabou Manta 115 which is 75cm wide not an Isonic, so you can take or leave my comments. I am lighter than you at 74kg and sail San Francisco Bay but I think my findings might be helpful, since the boards are almost identical in size.
I have been struggling to find the right fin for the board. I tried a standard 44cm Talon, it was great if I was very powered off the wind. When it was lite, which is was most of the time otherwise I would be on a 100 liter, it did not work for me with either an 7.6 or 8.5. So I got a 46cm Tomcat (yes Dennis will still make them) much better in the lite wind screams upwind which the Talon would not do, planes much quicker but drags off the wind. Recently I found out Dennis mills the Talons in several different foils thickness to cord ratios, the 9% standard which is very thin, 9.5% and 10%. I talked to him just this week about milling me a 9.5% Talon 44cm which will have a thicker foil so it should plane up faster and provide more power without sacrificing that slippery feeling off the wind the Talon is known for. I like the 46cm Tomcat for the really lite wind more likely in Florida than SF, but for powered sailing I think the modified Talon will work better for the top end of the board range. So talk to Dennis, for the price and consistent quality I don't think you can beat Techtonics.