There is no specific "right" shackle angle. Determining what is best for you depends on lots of things. Spring rate, shackle length, vehicle weight, arch, and travel range are among the big things.
But generally, for a compression shackle, on most vehicles, with typical after market lift springs, you will want the fixed end of the shackle to be somewhat closer to the fixed end of the spring than the shackle end spring eye. That means that the top of the shackle is angled toward the front of the truck. This will reduce your spring rate a bit, and is particularly useful if you have stiff springs like Rough Country. A longer shackle is also generally better due to the flatter arc of the swinging end, as long as you don't go overboard and cause problems for alignment and pinion orientation. But too much angle will cause physical interference with the frame on compression (or other limits due to geometry) and may lead to too low of a spring rate. There are lots of things to consider.
But the biggest problem typical of GM lifts is that the short front shackle is actually set so that the spring eye is *forward* of the frame mount, so that when you droop that side, the shackle lines up with the arch of the spring fairly quickly, limiting movement in the process. It also fight compression of the positive arched springs by effectively increasing the spring rate and jacking the body by forcing the spring eye downward as it moves back (as it must to compress the arched spring).
Searching for exact phrase "shackle angle" will get you lots of posts on the subject.
