My simple rule of thumb is to measure along the arc of the spring (main leaf), center of front eye to center of center bolt hole. Repeat the measurement for the stock springs. Subtract the two measurements and the difference is how far you want to move the center of the bolt hole on the new hanger forward of the existing hanger bolt hole. Why does this work? When the spring is flattened out, you are essentially at full compression and the tire will be stuffed up as far into the wheel well as it is going to go. Assuming you are happy with where the wheel is centered now with the existing springs, it'll be there with the new springs. I've used this with my 56" Alcan springs and it works every time:
