RE: iSonic 115 tuning (for Ian)
I guess the first comment is that the ST115 is not really the "ideal" board for tuning with a powerful 8.4m like the RS6. Yes for sure it's sailable with bigger fins and sails - but it's definitely taking the ST115 well outside the typical (6-7m) sweetspot range for that model.
Outboard slalom straps settings will be generally prefereable wth both these sail sizes (as well as matching the sail type to sailing style/stance) - in lighter and flatter conditions you will squeeze a little more top end from the board with the outboard rear straps, but ionce the wind and chop is up, one position FWD (Front and Rear straps) generally allows the rider to hold a more calming stance over the ST while still keeping a good amount of drive/power going thru the board.
Mast track for 7.2m would be typically 50-60% FWD, for the 8.4m the same, or a little further forward could help induce a little earlier planing, but in general don't go too fwd with the mastfoot to maintain a good combo of speed and free, responsive ride. If the board is really starting to fly away (too much fin/sail/chop) then moving the track forward more can help, but it's not really curing the problem efficently, rather just an interim dampener to get you thru..
Fins depends a lot on the ride style you're seeking; it's definitely valid to run race or slalom fins on the ST (especially when the sail choice doesn't really assist in SuperX or freemove style riding) and you wil definitely find the more upright race fins more efficent (for a given size), help a little in holding ST efficency when running tight upwind etc and/or marginal conditions and genarlly well behaved in speed in straightline, but the race fins don't quite maintain such an "easy" handing thru the jibes etc as the more raked freemove/freerace type fins. Sizes in turn vary a little with the fin (make / model) but for powered 7.2, a 34 is a good typical size (think range 32-34-36); for the 8.4, well I really try and keep a smaller fin on the ST115, so for that maybe consider to try a race fin around 40cm as a good allround option. 42 is definitely possible, but again well past the ideal sweetspot for ST115. (in a lot of ways, you are tuning the ST115 much more like an iS115/111, which would probably meet this sail size range better than the the ST).
Anyway, it's about having fun and hopefully you are getting plenty!
let us know if you want more info.
Greetz ~ Ian