Head shots on rodents is a great way to test ones marksmanship whilst doing a useful job. The overriding limitation is the rifle/ammunition group size at range. I have a intergrated supressed 10/22 T custom, for sub sonic .22 LR, for night shooting. The combination working range is 55m garanteeing a group less than an inch. I also have a Anschutz 1710, with high velocity .22LR, to go out to 80m. Reaching further and there will be misses. My Sako varmint 222 holds an inch out to 140m. My rule of thumb is that wind effects point of impact at about 2/3rds the "sweet spot" range. I've had a 308 in the past that wouldn't group tight at a hundred but did almost a minute of angle out to three and half; sweet spot 240m. Sweet spot is as far as you and the rifle combination can go without outside influences having any real inpact, matched to expected target size. A little input is still require, bullet drop, uphill downhill, but not too much. Target size can be a squirrels head or a moose's engine room but I always aim for a pimple.
One other thing, sights are often not aligned to the barrel, so as you add elevation you can also be adding some left or right.
I shoot factory ammo.
Do do some of your own tests and see what works for you. Atmospheric conditions, light and wind after about 400m doesn't allow for a true target picture. The target isn't actually where you see it. All you can try and do is to make the best shot consistently. Group size is then down to the rifle combination, your shot, and how you read the conditions.
The exception may be with benchrest 18lbs rifles, where wobble isn't a distraction. Lighter rifles and your body will be influencing what happens down range. Thats why I don't think higher magnification is any help. Shorter ranges you see what is actually there so you can take advantage of the magnification. In deliberate pistol shooting your focus is no further than the foresight, the target is blurred, but you can group tight.
I would be interested if higher magnification realy improves your long range groupings or not. I just think there is a limit to what is actually useful. Do tell us how you get on.