There is a wind tunnel test online, I don't have the link handy, of Fowler Flap testing on a P-38 wing model that finds the highest CLmax at 30 degrees flap extension.
The flap control "Maneuver" position was in the middle of the range of movement.
In Aces High the 3rd notch of flaps in the P38 has the handle in the middle and gives the highest CLmax.