Don't know. Thanks for the explanation of your join date.
Have you considered the difference in elevation of the end-points? You can get that off maps if you don't have an altimeter (
).
According to the experts you
should expect the tailgate down to be worse. It functions as what's called a base plate. A complete aeroshell (a specialized shape for the specific case of a pickup bed) is best, followed by a half-tonneau. The Search function is your friend.