Re: Usable range of a 4.5-14x50 scope
For an older model V3 I would say 14X in a 50mm is about the minimum for 800 meters. For the new models it may be better. So much of it depends upon lighting conditions where the animal is standing. Some of it depends upon you and the animal. If there are two deer standing together and one is a spike and one is a doe you may not be able to actually see the small horns and know which too shoot. Iif you don't ever shoot at spikes then that may not be an issue for you. Also background contrast, fog, glare etc all create issues. The genetic characteristics of a human being includes eyes and how they function, so different people are more or less sensitive to amounts and characteristics of light.
Sometimes more magnification alone will help but mostly the only things that help very much are lens quality, coating quality and objective size. Magnification helps refine the aiming point more than anything else. A rifle scope is a poor substitute for a good spotting scope but if you have a good spotting scope with you then you can get by with less magnification on the rifle scope. If the majority of your hunting is going too be in the woods and you need a low base power then just bite the bullet and stay with a 4X and let some of the longer range bad lighting shots go.
The most likely reason your search for a sensible answer has yielded nothing is that there is no sensible answer. It all depends upon your eyes and your comfort zone and your hunting technique and style.
The Smokin Fur Rifle Club