Unfortunately there is no way for us to know for certain "how many yards away this minnie would have to have been fired for it to have spent itself so fully." A civil war .58-caliber rifle's maximum range was almost a mile. But, if a soldier got hit a second or two before firing, and jerked wildly in pain, the rifle may have been pointing skyward when he pulled its trigger. So, depending on the angle the rifle was at, the bullet may have traveled nearly straight up, landing just a comparatively short distance from the shooter.
But of course, the more-often case is that the shooter was (unintentionally) aiming too high. Although the rifle's maximum range was nearly a mile, its maximum effective range was considered to be about 500 yards. And in fact, most civil war rifle combat was at much closer range than that, due to eyesight limitations.
We know that a lot of civil war combat was fought on forested or "brushy" lands. If a rifle is aimed a bit too high, the Minie-ball overshoots the target and comes down through a bunch of tree-leaves or brush --which slows it down a lot without causing the bullet to be "significantly" deformed like it would if it hit a solid object. For example, traveling through a bunch of blackberry or laurel bushes slows the bullet down a lot but doesn't damage it. It runs out of gas because of the leaves and thin stems, then plops to earth with little or no damage.
Apparently, such scenarios happened more often than we'd think, because in every can of 100 fired Minie-balls I buy from diggers here in Virginia, I find several that show little or no impact-damage.
Please pardon the long-windedness of that explanation. My point is, if the bluff you live on was forested or brushy during the civil war, your fired Minie probably was fired from less than 500 yards away, and got slowed down enough by vegetation to run out of gas and fall to earth without significant impact-damage.