You should tack it where you think you want it then set the weight of the truck on it and check your shackle angle. 90* or verticle is bad, but it shouldnt be laid back enough to allow the shackle to lock flat, or contact the frame. I have been told that 50* is a good angle to try for a good ride, and suspension movement. Your desired shackle angle will varry depending on what springs and you choose, and the length of your shackle. The shackle angle should be more important than the placement of the spring hanger.
