Thanks RotorFossil. It makes some sense that it was done for those early types with marginal performance. This would capture the Old Brantly B-2 I once flew. The practice of establishing a power margin with a minimum power fly-by and subtracting it from maximum power available gives a more real time prediction of landing performance than plotting graphs before departure anyway. Thanks for you perspective. I would appreciate any other opinions or experiences from pilots with exposure to the practice of applying humidity corrections in performance calculations.