Generally, 40 yards would be my max on whitetails. If I'm really on my game with shooting accuracy, and every single condition was perfect, I might extend that to 60. So far, since 1987, I've never shot one beyond 35. Places I hunt generally have so much thick brush, I'd never see many shot opportunities beyond 40. Also whitetails are much more jumpy than a lot of muleys I've encountered in the high Idaho backcountry.
I've passed on many small muley bucks I've walked up on, that just looked at me and went back to feeding, even at fairly close range. Some of them may have never seen a human before. So far, I've never killed a muley. The one year I had a perfect 50 yard opportunity on a 170-ish, 5x5, 26+" muley, I'd only bought an elk tag. Having seen those a number of times at a distance, I'll just keep passing on smaller muleys, hoping my lucky number will come up again. I'd take that same perfect 50 yard shot and maybe even out to 70 on one of those muleys, all day long.