ft.
The explanation put to me was a mathematical comparison of a free falling object with the weight of the aircraft versus the rate of descent of the aircraft in a spin.
Obviously, a fairly rough calculation but it fitted the theory quite well.
I do believe the nearly flat underside of a wing produces lift in the same way as a totally flat shaped wing would.
It would be pointless to curve the underside of the wing so as to create lift in the negative sense.