RE: Dihedral/anhedral effect on lift?
Harold,
Multiplying by the cosine of the dihedral angle should get you close. In most cases, I would expect the lift curve slope (increase in lift per degree of angle of attack) of a wing with dihedral to be a little steeper than for a wing without dihedral of the same span. This is because the wake of a wing with dihedral will be non-planar. In effect, dihedral gives some winglet-like benefits. Bottom line: I would expect the lift to be a little more than 0.94 at 20 degrees.