The only time mine has ever been at 90° was when I was testing it in a parking lot. But it gets to 45° frequently. I think it is pretty rare that I have ever been past 45°, and I think figuring 67.5° would be a pretty good compromise.
Having said that, don't forget that the front cap of your fiver angles back a lot and I would think you should have a lot more clearance than your drawing shows, at a height 6" above the cab where the kayak would be riding. The higher the rack, the greater the clearance.
Well, maybe strike that last thought, because the sidewall is vertical. Would still apply to a single kayak on center, but not outward from center.
You could consider going with a slider hitch, even though you have a long bed.