Gary,
I will review my manual and let you know if there are any valuable specifics, but I believe the recomendations are pretty general. They do convey the basic concepts though.
If the front of the deck is higher than the back, the material being cut will be cut a little bit shorter with each rotation of the blades, as you drive forward. That will shred the clippings, but the cutter blades will be constantly engaging material through the full arc of their rotation. Clippings will be shreded, but the constant contact of blades to material is horsepower intensive.
If the rear of the deck is higher, material will only be cut at the front of the blade arc, so less hp intensive. The greater the angle between front and back means a less smooth cut, best for brush and saplings. The greater angle helps keep you from contending with the little brush and sapling stumps more than once in a pass.
If the deck is level or the front just a bit lower than the rear, (more like a finish mower blade set up) the cut will be smoother...best for grasses, a manicured finish, or a final pass.
I don't think there is an exactly correct angle. With the basic concepts in mind, you can find an angle that suits you and your equipment and gives you the most acceptable result.
By the way, the uses for the specific set-ups mentioned above are just my own, and therefore, they are fair game for differences of opinion.
As a practical matter, I don't put too much thought into fine tuning brush hog adjustment most of the time. I just slap that baby onto the ground, set the front for the cut height I'm looking for, and procede with plenty of toplink slack, which means tailwheel is on the ground. If the results could be better, or the tractor is struggling, or I am approaching an obvious problem, knowing the basics gives me an idea what kind of correction to make.
OkieG