stay within your comfort zone, thats the main thing, but the more you shoot long, the farther your comfort zone is, been doing this long range thing since 1983. You practice enough at 750, get a method that works for you and 600 all of a sudden ain't that far. I still remember when 400 was a...
you practice at 750, and 600 gets to be a chip shot, have taken dozens at 500 to 800 yards, but this is whitetails,, but the kill zone on elk is larger. it ain't hard. put a good bullet with enough energy in the kill zone and stuff will die.
anything more than 9 or 10X on the top end is overkill. mine has a compact 3x9 on it, but its a woods whitetail rifle,, have other stuff for extended range.