A few variables to consider:
-Target size
-Color of target/background
-target moving?
-Type of position you are in
-shooting for groups or just an impact?
Shooting a big chunk of steel (20x40) at 1100-1200 meters is fairly simple at 10x when there is good contrast between the target and background. The more my position is "wobbly," the less magnification I normally run.
If the target is hard to see, or back in the shade, I run more magnification. White painted steel on snow is really tough to see in mediocre light.
If I am shooting for groups, I try to pick a small, precise aiming point, and stay in the upper end of magnifications to ensure I am as consistent as possible.
Sometimes at 400 you might need 8x, or even more. Sometimes at 1 mile you might only want 17x. I like using 15+ for checking zero, and find the higher magnification is useful when you are attempting to range targets with a reticle.