[aerohead]

On the long-bed with 32" wing and 48" tonneau they had a 16" gap. So it kind of 'scales.'
Your tailgate is much closer to the rear of the cab, so all the body proportions are different than the long-bed.
And Ford never published a Cd for the short bed F-150, so it's an unknown quantity.
Presently, the RIVIAN pickup has the lowest Cd. And it's cabin upper trailing edge to tailgate proportion happens to intersect the AST-III projection perfectly, so that's a clue for us ( GM's long-bed C-10 Pickup used in their tonneau cover research happened to match the AST-I contour. Their aero-shell, which Texas Tech also researched, would have also fit under the AST-I , their Cd 0.14 Aero 2002 concept was also AST-III, also their Cd 0.137 EV-1, 183-mph LSR ). They're smart folks!
I wouldn't be concerned with the gap.
That opening is part of the mechanism which transmits a lower pressure from the bed vortex core to the inner face of the tailgate, which is the 'secret sauce.'