There are two measurements that you can use to determine what spring length you’ll need for your project:
Spring Perch Hole Center-Line Measurement: If you are using an axle with vertical perch bolt holes, you can simply measure hole center to hole center of the spring perch bosses (be sure to measure from the top side of the axle), then deduct 5 inches. The resulting measurement will be the length you need for a spring-over application.
Shackle Pin Center-Line Measurement: If you already have your axle, perches, and shackles, assemble them and orient the shackles horizontally. Then measure from the center-line of one shackle pin to the other, as shown in the diagram below. This measurement will be the length you need for a spring-over application.
With the correct spring, shackles will be at a 40°- 45° angle when the spring is under vehicle weight load.