In order to get the proper distance to set the spring perch brackets, I set the spring on the table along with the shackles and the spring perch hangers and measure the distance from the mounting flange of the hanger on each side. I will often place a washer on the hanger on each side to spread the spring a little further. If after the car is loaded and I need some extra lift on the spring, I remove the washers and that will move the spring out approximately ¼ inch giving it more tension and a stiffer spring.
Here is an angle shot of the axle with the spring brackets in place and the all thread installed. The all thread not only aligns the brackets to the axle but allows the brackets to be centered from side to side on the axle. Remember to use the same size all thread that the holes in the brackets are otherwise you could misalign the brackets.
This picture shows a front on look at the alignment of the spring perch brackets.
Here is a close up of the actual way in which the bracket is held in place. A nut on each side holds the bracket in place and allows for accurate adjustment. So now you ask yourself, how do I get the all thread out after the brackets have been tacked in place? Two ways this can be done. One, is to simply cut the all thread in the middle and pull each end out. The other way is to use two pieces with a coupler in the middle and just screw the two halves together and lock in place with a nut or two and then do the adjusting at the ends.