Shackle & Spring Angle Relation

Ultimately, the shackle and spring should should for a single line. But that's generally not going to happen perfectly, it's going to be more of a curve as the spring bend sharpens. Get as close as you can to having the shackle line up with the spring at full flex. Figure out what's limiting this by checking shackle bolt torque (bound shackles due to overtightened bolts usually indicated that the metal sleeves are too short, or word down, allowing the bushings to be sandwiched between the shackle plates as the bolt is torqued.) If the metal sleeve is the correct length and not worn down, you should be able to tighten the shackle bolts to the manufacture specs of around 80-90lbs without loosing flex.