Fokker-Jock
[pedantic]
I'd just like to point out that for most flapped gliders, the "absolute best" L/D is acheived with 0 deg flap settings, when the wing is in its "cleanest" state.
Negative flap settings are indeed used to increase the L/D at high speeds, but the max L/D for a given flap setting is generally lower that the max L/D you get at 0 deg flaps (at a lower speed).
more
here (with polar curves which should make things clearer)
[/pedantic]