Hi Raggy, a year ago I switch from a carve 90 to the st-93 and that was immediately a succes. The board has a broader range and that gives more sailing time.
Indeed you have to try several settings and find out what suits you best. I can share my experience and maybe it can help you.
I sail with the foodstraps on the inside, because of the better control in chop. Depending on the sail size and wind, but my average mast position is 1/4 from the back of the board.
The fins I use are from select. I have the s-ride an a supercross fin.
a 28cm supercross for my 5.0 (NP search) and sometimes my 5.7. Supercross is my favorit fin, maybe not the fastest but it's more fun and feels good in more choppy conditions.
Normally I sail the 5.7 (NP saber) with the s-ride 30cm, works good too. I tried a 6.6 (NP spirit) with the s-ride 30 cm and 34cm, probably a 32cm would be better for me. My weight is 75kg.