The target is any spring that will survive your bike. How many springs have you broken?
Only one, on my warranty replacement gearbox. The original box went 26,662 miles with no spring trouble. Warranty replacement box broke a spring just shy of 27,000. That was a tick over 51,000 miles ago . . .
Maybe it's just this birthday thing, but at this point, a 372,000 downshift rated spring would have me all set.