Is there any aerodynamic benefit to sloping from the center fer the rear part of the cap? It's doing it naturally because of the lack of support...but should I build it that way or should I put in a support beam to make it straight?

Just did the math with my measurements.

Using

this online trig calculator...

Here's how I calculated it...

First calculation:

Opposite = 21"

Adjacent = 78"

Angle (calculated) = 15.07°

Hypotenuse (calculated) = 80.7696"

Since I have it 11" higher...

Opposite = 11"

Angle = 15.07°

Adjacent (calculated) = 40.85"

Hypotenuse (calculated) = 42.3079"

I add the calculated adjacent side to the original to git 118.85" and plug that back in to calculate the actual angle with the 11" rise from the tailgate.

Opposite = 21"

Adjacent = 118.85"

Angle (calculated) = 10.02°

Hypotenuse (calculated) = 120.6953"

If the top ended at the tailgate...the angle would be about 15°. Since I have it up 11" from the tailgate, the angle is about 10°.