If you are shooting factory box ammo and it is cycling OK as is, no need to change it. I think I went down two pounds from factory in both the 75B and P-09 for use with the lighter match ammo loads from Atlanta Arms. I am not a run and gun type shooter. Some folks go softer on the springs than I will. I tend to use the heaviest spring that will allow the gun to cycle reliably, just to make sure it locks up exactly the same for each shot. Run and gunners are wanting to tune the gun to their grip and second shot timing, so tend to run lighter springs than I will for slow precision shooting. If the ejected case will land between 4 and 6 feet away, I'm happy. Light springs may result in ejection distances of 8-12 feet? But the slide is easier to cycle by hand with a light spring also. The light springs are harder on parts stopping the slide's motion to the rear also.
So "best" is dependent on the gun usage and ammo choice. The factory spring works great (from what I remember) with most box ammo after the new gun loosens up a little, which takes 100-300 rounds. After that, the factory spring will usually work well even with softer box ammo or go at most 2 lb softer if the slide doesn't catch the next round when cycling with the factory spring.
Joe
