200 yards. Think Caterpillar and 25-30 wide. Why: Necessary for sighting in purposes. You'd be surprised to look at a 20 ' backstop/berm and how little safe area it offers from 200 yards away. Better to be a little over-size on your berm than have a high power bullet get away from you down range. Just my unbiased opinion is all.
"Any rifle will shoot bullets through the same hole. You just have to be able to see well enough to aim right."