If you are in full control of the rig, and the board begins to get hard to control, move the mast foot very slightly forward. This should calm the board a bit.
Or, better yet, change to a smaller fin and leave the mast foot at the sweet spot.
The sweet spot may change slightly as you change fins, and tune your rig for more wind, but not much.
It's all about balancing things out.
If you have more wind, use more downhaul and outhaul to reduce the power in your sail.
If you are having trouble controlling your fin (i.e. the board is "tailwalking" or you simply cannot keep the board going straight) changing to a smaller less powerful fin can make a huge difference.
Hope this helps,