The pilot training manual does say maneuver flaps is approximately half but it does not specify half deflection.
My experience with Fowler flap systems (quite a bit) is that they extend much more than they deflect and the pilot training manual reflects this.
It specifically says the maneuver flaps or 1/2 flaps provides increased wing area and provides greater lift. Beyond half, they act as airbrakes.
In general, with ALL flap systems, the first 10 degrees is almost all lift with almost no parasitic drag (there is an increase in induced drag). With Fowler flaps extending aft, increasing wing area as the first 10 degrees ( most Fowler systems are 8 degrees or 12 degrees for the first detent), the lift increase is very dramatic.
Once you get past that first 10 degrees of DEFLECTION the flap add lots more drag than they do lift.
The P-38 training manual is in line with the aerodynamics of that situation.
Maneuver flaps on the P-38 (as verified by the Red Bull pilot) is 8 degrees DEFLECTION and 50% EXTENSION producing boatloads of lift with very little drag penalty.
Flaps deflection past that is almost all parasitic drag with a smaller component of additional lift.