So much "depends."
Number of potential shooters tends to set the width.
Available terrain is a factor.
Ok, some ideals:
Orientation: Shooting position to targets ought to be SW to NE.
Elevation: Shooting position ought to be higher that target (if only so that drainage is away from the shooting position). Ideally, the target area ought to be a tad higher than the lowest part of the range (so drainage is away from target area).
Ideally, the change in elevation is less than 5º (a 2%, e.g. 1:50 suffices).
Berm height: Really, twice what's needed. Ergo, if 6' works, you want 12' high.
Berm geometry: if you hold the slope of the berm to 40º it will grow grass, and grass is yout friend for holding a slope. If you use something else, tires, railroad ties (sleepers for our Euro readers), or the like, those materials will be worn away by shooting, and will need replacing at some point. Planning ahead for that replacement costs you nothing now (unlike picking creosote covered splinters out of a backstop, your gloves, etc,)
That's what I can offer for 2¢
In doing the physical layout, what you want is a helper and a rental place to set you up with a dumpy level and a hundred foot tape.