I'd maybe try setting the angle with the absolut most drop on the suspension you are likely to see, just to make sure that it wont be too crazy. If your operating angles come out to be 3 degrees or less where you are sitting now, you'll likely be fine though. I would maybe set the pinion a half to 1 degree lower in the front than you need it, just so that when the pinion rolls up under load it is perfect. Or are you going to a 4-link? If leaf springs give some wiggle to account for spring flex under load, if 4-link then it can be dead on because you shouldn't get any rotation of the axle at all under load with a 4-link.