It isn`t. It`s more complicated because interthermal speeds doesn`t
vary with the square root of % extra weight. In condor is a nice tool
which shows this very clearly.
Reality shows that filling a glider meens flying "a bit" slower than
sqrt(% extra weight), increasing range and the change for a real good
thermal.
The extra gain depend almost fully on the diameter of the thermal, with
1 m/s and huge thermals you can fly a racing glider almost full. And
what about the gain running cloud streets? Also not linear.

For huge thermals and weak cloudstreets of 1 m/s actual gain for an
Diana would be (theoretically) 40%.

Assuming 3 m/s in typical European (small) thermals shows a gain of
only 8%.

Life`s complicated ;-)





l/d max occurs when induced and pressure drag are the same, not at 75%
induced. (interference drag is (per definition) negociated)

When 75% of the drag is induced you`r flying at Vy-min (min sink)

One other point: the higher stall-speed is worth mentioning because of
difficulties with landing, thermalling, manouvrebility etc.