Thanks a lot for all the answers! They were very helpfull...
My conclusion "on the paper" so far is:
4bft -> 137 with 9,7 Relfex II or 10,0 RS:Racing Evo III, 54 cm fin
5bft -> 137 with 8,6 Reflex II or RS Evo III, 48 cm fin
6bft -> 107 with 8,6 Relfex II or RS Evo III, 44 cm fin
7bft -> 107 with 7,0 Reflex II or RS Evo III, 38 cm fin
Of course the setup may slightly vary due to how constant the wind is and how big/choppy the waves are.
With this setup I can still think about adding low wind range by entering the formula scene with corresponding board and 12 m2 sail using also the 9,7/10 sail, and adding high wind range by buying an isonic 90 or even an iSonic Speed W58.
Maybe I will even diversify inbetween, if I get keen on hitting the Slalom competition scene :-) But all this is future and "evolution"...
I still have two more question:
1. Is there a noticeable difference between the two sail brands mentioned above (Severne Reflex II and Neilpryde RS:Racing Evo III)?
2. What is the difference between the Reflex Slalom 9.6 and the Reflex Formula 9.7? Which one do you recommend for the iSonic 137?
Further comments are very welcome!
Thanks and cheers,