OP,
Greg identified what's happening with your sight picture. A mirage band stretched from muzzle to receiver will solve that problem.
Back to your question of how much power, first consider how much power is needed to understand where the barrel is pointed. Using center of mass holds, the natural ability of the eye to both center and balance things may make magnification moot, if the target is big enough for you to recognize a consistent sight/target relationship; but, since you now have a mindset telling you it's magnification which is what allows a precise understanding for where the barrel is pointed, it may take a leap of faith to come to understand that the best results are also possible with no magnification, when the target overall is big enough to be seen at the distance you wish to shoot it. One more thing, although magnification can be used to see small objects such as golf balls at extreme distance, the capability to hit the target does not increase with the application of more magnification than what is required to see the target, just your ability to see the target better.
Here's an idea, turn your power all the way down and aim center of mass on something like the MR-31 target. This is a 600 yard HP target reduced in scale for 100 yard shooting. After you have altered your mindset that all that is necessary is to understand a consistent sight/target relationship, which does not demand magnification, you will come to realize you can get the best results at lower magnification, or with no magnification; and, you may also discover that higher magnification is in many cases just adding distraction to good shooting.
