Full batten sails as pictured by Navigator are
unreadable as far as set goes when it comes
to light wind conditions.

The battens hold the shape of the sail even if
the angle of attack is wrong, even if the draft
is wrong, even if the twist is wrong, and even
if the sail is hoisted upside-down.

To learn how to properly trim a mainsail one
must get a proper cruising mainsail such at
that shown on my website.

http://captneal.homestead.com/newsails.html

One must pay attention to the tell-tales. Note
how the two uppermost tell-tales on the leech
in this picture

http://captneal.homestead.com/files/mainsail.jpg

are backwinded. This is the correct way to do it.
If all tell-tales are streaming aft in light winds
then one is not sheeted in enough or one has
the leech too slack. The upper two tell-tales
are backwinded by the beginnings of the tip
vortex which indicates max lift/drag for the
light wind conditions.

S.Simon