I've just recently purchased a scope with variable magnification (5-15).

Up until now I have only been shooting paper at 15 m, but I have a hard time deciding which magnification I prefer. Diameter of the target is 47 mm (1.85" for you imperial guys), and with a magnification of 15x the black part of the target spans two mildots in my scope.

However, aiming sensitivity also increases with increased magnification, and while it is nice to have this very detailed image of the target, I tend to dial a bit down on the magnification to lower the sensitivity. My groups leave a lot to be desired at any magnification (as you can see from the target above), but I suspect the pellets are partly to blame (have ordered a couple tins of JSB Exacts at 4.52 to try).

Are there any rules of thumb for choosing magnification? For now, disregard that the scope is only true mildot at one magnifcation (15x in my case).

Regards
-Øyvind