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.
"Do not follow where the path leads, rather go where there is no path and leave a trail for others to follow."