OK - you understand that if a sail hooks that is bad

Therefore a sail that has a SLIGHTLY open leech at the top is good. If the top is slightly open the rest of the leech all the way down to the boom on a correctly cut sail will not hook to windward.

I prefer to make sure that the top will open early by having the top batten stiffer than the others. In light weather (<5mph say) you want the sail twisted slightly - achieved by little downhaul tension, and little sheet tension.

In a medium breeze you want the top working and the leech standing up to allow you to point higher. Fair amount of downhaul and loads of sheet tension.

In a really good breeze you want the top to flare in the gusts, so loads of downhaul and as much sheet tension as possible without being overpowered. BUT if you are being overpowered then let the traveller off-centre (2-3 inches should be enough) and don't point quite so high - go for speed. At this level of wind the stiff top batten should be flaring automatically in the gusts - 'gust response'.

Hope this helps.