Leech tails in the upper third help avoid hooking the leech to weather from over sheeting. You want the upper leech tails to tuck / fly / tuck etc. alternating.

If they are tucking to the lee side all the time, the leech is hooked to weather, sheet out. If they are flying all the time, you are under sheeted, sheet in.

Light air... sheet out a bit to open the leech. Travel more to center and sheet out a little.

Downhaul causes the leech to open, so downhauling less and sheeting harder should get the leech tighter and make the tails tuck / fly / tuck more.

It is mostly a balance of sheet, traveler and downhaul... but mast bend and rotation some too.