My drawing program measures a spring from the blue crosses on the drawing, this would be correct if the ends were ground like the image. I had to jimmy the distances as I measured from the ends and wanted the drawing to show this.
If you want the strongest springs available for an R3 just call Carpenter and get a set of their springs. Stronger than Barnett. Why risk any clutch slip?
This is what the locally made spring was in our tests. I took the Barnett springs down to a place where an old couple make springs and had them make me two sets to the same, most of the cost was in the set up for the springs. They perform nearly identical to the Barnett but are not as nicely finished. Didn't really need them but have another Rocket in the shed that I may change out to these springs.