Fin selection for flat water tracking (big cruiser)

[upwinder]

Noob here - newly on a Starboard Atlas which I'm liking very much. I've been experimenting a little with trim. The board comes as 2+1, the stock centre fin is 8 1/2" x 5" sickle shape with a fair bit of rake.

The board has a really nice glide for 33 inches wide but I'd like to make it track a bit better in flat water and light chop If I can (2-3 more paddle strokes each side would be nice). I've taken the side fins off and set the centre fin back a bit - seems to have helped but am I right in thinking I could do better with a bigger/different single?

I've seen crazy long "downwind" fins up to about 15" with narrowish bases, "race" fins around 10-11 inches with wide bases and more of a sickle shape - what's going to get me better tracking while maintaining good glide?

(I've used the search function and read Pono Bill's artice but I'm still a bit in the dark)

[upwinder]

...I'd like to make it track a bit better in flat water and light chop...

what I really mean is track better under aggressive paddling, so I don't have to reach and torque so much to correct course when giving it some beans