Children, stop bickering.

Simon, As wouter says, fat top sails to not respond well to cranking lots of mainsheet on.

Having sailed a Spitfire around the IOW a couple of years ago in various conditions I have to agree with Wouter, you need to use Downhaul, mast rotation, traveller and mainsheet. Quick and simple guide.

Very Light wind.

moderate mast rotation, bit of downhaul to flatten the sail a little, moderate mainsheet tension (to try and keep the leach telltails flying)

Light wind.(only one or none trapping)
Max mast rotation and mainsheet tension to keep the leach telltails flying.

moderate wind (2 trapping)
move the mast rotation back a little to allow the leach to stand up some more, more mainsheet tenstion, increase downhaul and mainsheet tension as windspeed increases.

higher wind
More downhaul, more mainsheet tension (but keep the leach tell tails flying), traveller out a little in the higher range.

Very windy
max DH, ease traveller and play mainsheet.


At some point you may also want to move the plates up a little.

Fat top sails need care toget the best from them. Oversheeting kills them.