To the original question, I say the magnification depends from your stance, your scope and the size of your target.
I have Howa 1500 Compact varmint, 20" .308win barrel. Leupold Mark4 3.5-10x40 LR/T on top of it. I shoot it from the bipod rest, from the prone stance. Nothing else, excpet the shooting matress. I mostly shoot IPSC targets and animal figured, silhouette falling plates. Both are quite large.
Longest range I have shot my Howa + Leupold combination is 550 meters. I like the best magnifications for the ranges up to 550 meters to be x6 or maximum x8. Smaller magnification gives you also an advantege to monitor the area surrounding your target more better and take notes to help your aim or impact.
Why? Because of the factors mentioned above. When I use the x10 magnification, the sight picture comes a little soft and foggy. I prefer the x6 because the picture stays very sharp and crisp. The x8 is also good. Also when I use bigger magnification, I see more jumping and moving on the crosshair, which makes me more unsecure, nervous and lowers my "self esteem" to make precise shots. When I see the wandering or moving in the crosshair (because of the high magnification), I start automatically to hone and move, trying to make my stance more "better". Usually this results a more poorer and unergonomical stance, which makes me shoot more worse. For me, smaller but reasonable magnification means more relaxed shooting and tighter groups. I think the stance also plays a big role for choosing the magnification. More stable stance, more bigger magnification you can use.
With x6 to x8 magnification my Howa shoots on a good day about 1 MOA groups, little more or little less, to 550 meters with handloaded ammo using Lapua lockbase or Nosler CC bullets and Vihtavuori powders. That means 15-20cm groups.
For the different ranges I recommend from my standpoint:
under 200m: x4 or less
200m-400m x4-x6
400m-600m x6-x8
Smaller targets will naturally need bigger magnification.