There is really no standard for too close. I have had elk just outside of my tent. When camping out in the open, I have had them walk by at night only a few feet away. That said, I would prefer to be back far enough so I could move around and still not disturb them. Terrain and vegatation are major factors to consider. Being over a hill with some timber in between you and them could be enough to not be detected. I can usually find a good place to camp within 1/2 mile of elk where I start to hunt, but remember elk move. I guess you just have to determain how important it is not to risk disturbing any elk. I kinda like to hear them bugle at night and if I bother a few; oh well.