Good advice from Guy.
However, I strongly feel the root cause of the spin-out on the STBs is the low quality of mass-produced fins. Yes, the STBs are a critical sort of design. They are outrageously playful, skatey and great for bump and jump, some freestyle, and light wave riding. The board qualities and geometry, including fin and footstrap positions, that brought this fun also put more demands on the fins, however. With the economics of production fins - meaning low cost - come generally mediocre quality and performance, and thus, spin-out. Fanatic is not alone in this regard, BTW. All board manufacturers have this dilemma - how to get a complete board with fins to the buyer at a reasonable cost. The STBs were just a more fin-critical design.
We own all four STBs - 85, 95, 105, 115 and have used them since early 2017. My daughter and her husband, who sail in SF Bay, have three of them - 85, 95, 105. We all love them. We sail our STBs with three fins and with singles, depending on conditions. All kinds of conditions from flat to bump and jump, really rough water in SF Bay and La Ventana, and small waves on the Gulf and Atlantic.
The key was to get some REALLY GOOD FINS. In the US, Tectonics Maui fins are exceptional. We use Tectonic's B3 center fins in 20, 22, and 24; and their Beastie side-biters, generally 9 and 10 cm, occasionally 11 cm. The Beasties are a very unique design, from CNC-NC in Hatteras, whose design Tectonics machines to precision. Very sophisticated, and head and shoulders above any sides we've tried, and I've tried many.
The production fins that came with the boards are not very good. The single 30 cm from the 115 STB (Textreme, no less!) was particularly miserable and spun-out constantly. All those problems go away with the TM fins. Their Falcon fins are also great for rough water and work well as singles. Just go a little larger than normal on these since they are a slender design and benefit from some length. Slippery and fast, but no spin out. Call and talk with Dennis at Tectonics and he'll give you some solid recommendations.
Tectonics are precision CNC fins, thus pricey, but worth every penny. We are not sponsored by anyone, BTW, we just enjoy quality fins.
For the examples Guy mentioned, 4.7 and 5.3 on the STB 95, the B3 in 20-22 work well, along with the 9 or 10 cm sides. For a 6.0, we'd use the 22-24 with the 9 or 10 sides. As he suggested, you could go to the larger side of that. BTW, I am 80 Kg and 182 cm.
If you do a search under my postings on STB fin topics, you'll find a lot more info, particularly with regard to weed fins, which are a similar problem, and harder to solve. Happy to help with other questions. Good luck!