Re: 12X for 750 yards
I just went through the same scenario (deciding what was adequate - only for 1000yds).
In order to help me decide, I set up a couple of my existing variable power scopes on my back porch and looked over the pastures behind my home where I had good clear lanes of vision out to about 900 yds where the closest tree line is.
My debate was whether a 10x was enough for a 1000 yard hunting shot.
Bottom line after zeroing in on "cow pies" at about 700 yards, is that the 10x was absolutely enough glass to do the job. It in fact would be adequate out to 1000 yards for game size animals. (Benchrest shooting of course is another story). At that juncture, I decided that a variable scope up to 10 or 12X would be perfectly acceptable.
Is more better? Maybe, but I don't think it's "required" or in some instances even desired. Once you start getting up to the more powerful, higher quality scopes, you are paying the price with considerable added size and weight.
The only caveat to what I said above is that the quality of the optics has a lot to do with what you can or can't see at a specific magnification. Get the best quality glass you possibly can in any case. A smaller crystal clear picture beats a larger, fuzzy picture EVERY time.
Suggest you take a decent variable power scope with all the potential powers you are considering and sit down somewhere and do your own evaluation. I think you'll be surprised at what you discover. But at least you'll personally know what works for you.
Hope this helps. Good luck.