In theory, you never want the shackle past vertical towards inversion with the weight of the vehicle on them at rest. This would mean that the spring would be tring to lift the weight of the vehicle to get the shackle to extend. not only does it ride funny this way, but it can easily invert.
The other posters are correct, the shackle anlge is dependent on many variables. Some of these are: weight of your vehicle, the angle the spring is mounted on, the location of the centering pin relative to center, spring rate, spring length, and the ammount of arch in the spring. You really do need to have the thing mounted withe the weight on it to get a baseline idea of what you are dealing with.
Now for the issues with your particular vehicle. The way the shackle mounts to the framerail in your truck, the spring can't extend very far at all without hitting the bottom of the framerail. So you can't install a longer than stock spring (eye to eye distance) or it will hit the frame unless you run longer shackles (which changes pinion angle, ride etc), or fabricate an entirely new setup. The lift spring companies tend to just add arch to stock length springs, and you wind up with your shackle angle incorrect. Unfortunately, you're going to have to deal with it unless you want to build something new.
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