If you really want to do some fine tuning, collect some fiberglass scraps, lay up some fin panels and position your tabs fore/aft for minor position changes. There are lots of 'how-to' threads on fin building around here someplace.
One thing you can do when you install fin boxes, especially those FCS plugs is to use fiberglass cloth scraps to make a slurry. I cut the left over glass from laminating boards into pieces large enough to make fins. I cut the sections that go along the rail that are narrow into small chopped up pieces, about 1/8" in length. Just take scissors and cut the glass up. Then I add that chopped glass to the resin to fill the void where the holes meet the deck. Use a small stick or toothpick to push it down. The glass slurry will be much stronger than just using resin, or resin with micro-balloon filler.
I use to make an entire patch of wetted fiberglass around the plug, press home and fill with resin, but others prefer milled fiber or similar. Somewhere I read that filling only with resin without fiberglass is more resistant, maybe because of the elastic nature of epoxy without hardening with the reinforcement. Never did test panels or similar but never broke a plug, leash or fin installed this way. By the way I dont like very much removable fin systems, like the feeling of a one piece board.
Milled Fiberglass is a lot less sloppy and messy.