Frank,

As you fly the glider more, you'll get a feel for the best flap
position the way Bill describes. The chart in the flight manual is the
best starting point, I used to use little laminated versions for
different wing loadings for my Ventus 2C, but after time found myself
just going by the feel of the handle.

Before you do any of that though make sure your weight and balance is
current.

~ted/2NO

ps don't use the L setting for landing unless you really have to. It
puts excess stress on the mylar seals.