757/767 has almost exactly 50% reduction in stall speed for the flap movement.
However, we operate in 20 kt speed reductions allowable per flap configuration, which is not the same thing as the actual reduction in stall speed.
Are you positive the you're commenting on actual stall speeds vs. SOP speeds per flap configuration? They're two different situations. I'm asking because a 747 classic pilot told me that the stall reduction for the first notch (or two?) of flaps on the 747 had a similar impact(high percentage of total stall speed reduction).
I don't know where I'd find stall speed vs flap configuration published for the 747 (or any other transport airplane, for that matter). Therefore, I can only assume that flap maneuvering speed is based on a consistent margin above the stall speed for that configuration.
The "or two?" comment you make, however, is also relevant. The second increment of flaps on a 747 gives a total of 40 KT reduction in maneuvering speed, of the grand total of 80 KT reduction from clean to Flaps 30. So, in the case of the 747 it IS likely accurate to state that 50% of the stall speed reduction is attained in the first TWO "notches" of flap. Likewise, 75% is attained by the third "notch," with 3 "notches" remaining.