RE: free formula 158 vs 155 isonic????
Especially because of it's "slalom" orientation, rather than FW etc, the larger iS boards are supplied with a medium (maybe "average" is a good way to describe) size/range fin (relative to their board size/intended range), rather than the fin that gives the best bottom end.
So it's logical that especially a heavier rider can add some marginal conditions/lighter wind performance to the iS155 by tuning with more fin ; for a 90kg rider and 10-11m sails, take a race fin in the range 60 to 65(max). If the wind comes up a bit when you are out on the 10m, then it would be OK to change down the fin to the stock 56 (as a half step before having to change down the sail to 9m). Obviously the ride will stiffen up a bit, the slalom feel/jibe etc will go further towrds "free formula" feel - but you'll have more bottom end performance overall- which is what you want in those conditions.
Also in general, try and refine the early planing technique to get the board speed up a little bit higher before (progressively) loading the fin with load (increasing the load also progressively, with board speed). This might sound a bit like the "chicken and egg" situation (cant go faster until you can put on load) but with a little practice, it's a good technique to develop. Not just for maximum size gear, but for getting smaller gear going even in (marginal) higher wind conditions. A further tip here is to avoid trying to pinch the board up high/tight to the wind too early (in it's marginal planing speed range) - rather keep the board headed a little bit broad/downwind (just a bit) as you accelerate, then as the speed comes up, the load can come on the fin (more) and your angle can come up higher towards the wind.
Cheers ~ Ian