Can't be bothered typing. From a mag article:

Once upon a time, every board had a "classic" finbox, allowing quite a range of adjustment of fin position. Nowadays, with most boards having powerbox or tuttle fins, the whole concept of adjusting fin position is something that many windsurfers don’t even come across until they get their first wave board. And then it’s usually a case of, er, ah well, stick the fin in the middle of the finbox, like with the mastfoot... OK?
No! Fin position has a major effect on a board’s performance. The basic rule is - bringing it forward loosens the board up (but can make it more prone to spin-out), while moving the fin back gives more tracking, speed and control. And there’s nothing wrong with going to the extremes. The board may work best with the fin right forward or right back in the finbox!
Unfortunately though, beyond these basic concepts, it will come down to trial and error. But it’s worth experimenting with as even very small adjustments can make a big difference. John marks his finboxes and tweaks starting from what has worked on previous boards, while Chris finds having the leading edge centred under the back strap gives him a gauge and he tweaks from there. As for Mr Goya:
‘Further forward in onshore wind (to increase snappiness). For frontside a little further back for control and speed.’

Fin back also helps going upwind, in light conditions i often move my fin back to help keep upwind and vice versa, otherwise marks info is spot on.