My longboard has a large fin box for a big centre fin but were should I position the fin ?

Once you insert your surfboards centre fin into the fin box you can slide it to the position you want and then tighten it in position ready to be surfed. So where to position the fin ?? If you push the fin as far back as you can this will maximise the effect of being able to keep the board tracking in a straight line. If you push the fin as far forward as you can in the fin box this will maximise the ability to rotate the board around the fin so giving more manoeuvrability. Any position in between will give a varied balance between the 2 extremes. Its well worth experimenting with because it will make you surfboard behave in a noticeable and different manner !