One thing I can add is to take into account 'compression', settling, etc.
We bought a little more than 40+ acres, in the woods basically, and had about 3-3.5 acres cleared, for the house and basically for a "yard" for my (now-passed, but perpetually beloved) 4-legged-son ... and the professional excavating service we hired had specific instructions ... a certain amount of the trees and Saw Palmetto bushes/roots/etc. were to be used to build up a backstop for my firing range. Well the guy in charge was a firearms fanatic and took to the job with gusto. Within a day or two I think it was almost 30 feet high, extremely thick at the base (obviously got thinner as it got higher, like one of the pyramids) and quite wide. It was great. Well, I think it's down to like 16 feet. I'd say it was about 8.5 years since it was built, and it has settled *tremendously* ... no big deal, still more than adequate and every once in a while, my Dad - who likes to play with his tractor anyway - will "backfill" stuff he's dug up randomly onto the back. But the point is, it settled and got smaller and smaller on a significant basis. No big deal if you start waaaaaay more than you need... but if you make it juuuust enough, you might find down the road, it's no longer capable or safe.
And that brings me to the safety lesson of the day - I know everyone here is a professional, knowledgeable, and safe shooter- but on the off-chance someone reading this doesn't realize - you're responsible for every round you fire ... Contrary to what some believe, it's not "OK" to just "try your best" ... When we moved down here, one of the neighbors joked how he hopes he doesn't hit our house when he shoots - uhh, what now?? "Well yeah we go out on the back porch and shoot into the woods, and we generally try and hope that we hit the trees ... hopefully none get through everything and get down your way! But hey, we try ... that's all you gotta do ... all you can do ... you just gotta TRY not to shoot someone or something, and you'll be fine... "
To this day he and his wife don't seem to understand, that's completely false ... My Dad as a (retired) cop and me as a law student tried to explain ... Every round you fire "has your name on it" ... First-day-law-school-students are taught the difference between "Murder 1" (1st Degree Murder - Premeditated Murder) / "Murder 2 (Murder in the 2nd Degree - you didn't go to the dude who's cheating with your wife's house to kill him, but he mouthed off and you then decided to shoot him in the heat of the moment...) / and what would be called Murder 3 but is popularly known as Manslaughter - and one commonly-used scenario to explain Manslaughter is, you're out back shooting a legal gun on your own property, but you don't have a proper backstop or have taken adequate precautions &/or are shooting negligently or in an unsafe manner or you just didn't have the right "range" in your yard ... and one of your bullets accidentally and inadvertently leaves your yard, goes somewhere and hits & kills some innocent. You absolutely didn't mean to kill them, but you were doing something you shouldn't have been doing - in this case, shooting unsafely, - and someone died as a result. Every round that leaves your firearm "has your name on it" ... They honestly just couldn't accept it ... "Nonon ... as long as you try , everything's fine ... it's not my fault if I shoot randomly into the back yard and someone somehow gets hit ... I wasn't aiming at them!"
Yeah, No. Safety Reminder Of The Day: You own and are accountable for every round you fire. Thank You & Have Good Day.