It's not the radius, it's the whole axle diameter, plus the thickness of the spring pack, plus the height of the original bottom spring pad, plus the height of the new top spring pad.
You're moving the bottom of the spring pack from where it is, about 3" below the bottom of the axle tube, to where it will now be, about 1" above the top of the axle tube. This example is assuming both spring pads are about 1" high and the spring pack is about 2" thick and the axle tube is about 3" diameter.
Going from bottom to top, it's the spring pack thickness (2"), plus the bottom spring pad height (1"), plus the axle tube diameter (3"), plus the top spring pad height (1"). This totals 7", in this example. Axles can be all different diameters. Spring packs can be all different thicknesses. Spring pads can be all different heights. So it is entirely possible to end up with a measurement less than or greater than this example.
Another example could be a 2-1/2" axle, 1/2" spring pads and a 1-1/2" spring pack, which would make 5" instead of 7".
