I doubt there is a "magic spot" across all glider types.
I would say, in general, pick the speed you will spend the most time at in a certain contest, then fly your normal stuff and see if the flying speed (no trim) matches. This means chute, snacks, water bottles, clothing for the ambient conditions, etc.
Adjust CG to allow "0" elevator (meaning chord of tail and elevator are even, thus no lift in either direction) so you can normally fly no trim.
This is least drag.
In engineering, as in life, there are compromises, fix one thing, likely to FUBAR another.

For most of us mere mortals, the biggest difference is the nut behind the stick.........