As far as where to position your mast foot, that depends on what board you have
(hmmmmm.... what board are you sailing?).
Formula boards run the mast foot way forward, to get better VMG upwind, but this
can cause issues downwind as they run so deep.
On slalom boards, you might want to start nearer the back of the track (vs the middle) unless you have some marking or other compelling reason to put it more forward.
Near the back usually gives you the best ride at higher speeds.
Put the mast foot too far forward and your board tends to pound and rebound.
Moving the mast foot back a little can really smooth things out.
If you have it too far back and you are really going top speed and are overpowered, the board can begin to have control issues (called tailwalking) and you need to move it a little forward to settle the board down, or reduce the size of your fin.
Hope this helps,
Last edited by Roger; 16th June 2009 at 08:29 AM.