I assemble the axle under the rig with the perches not welded and the U-bolts loose enough to rotate the axle.  With the weight of the vehicle on the springs and the driveshaft in place you can use a jack to push the pinion up until the lower U-joint of the driveshaft is perfectly straight.  I then remove the drive shaft and measure the angle of the pinion flange with a magnetic angle finder.  I then bump the flange back down 2° and that is the angle I want to keep.  I tack my spring perches firmly in place and remove the axle for complete welding.