I' ve always heard that for elk the minimum energy @ impact should be 1200 ft/lbs and for deer should be around 1000 ft/lbs. I prescribe to the ElkCampMaster theory of never going undergunned. I shoot a .300 mag when elk elk hunting and set up my loads so that I am maintaining at least a ton of energy at any range that I will shoot at. That puts me out to about 475 yds. I simply won' t attempt anything longer that. Not only for energy reasons, but also for reasons of bullet drop. I set my gun up for an optimum point blank for a 8 inch target. That puts me on a 300 yd zero and a 370 yd max. With this set up, I can shoot out to about 475 yds and still be able to keep my crosshairs on the animal.

The long and short of it is..It' s good to think about impact energy, but IMO you need to think as much about getting the package there as you do about the the size of the package. There are a bunch of FREE ballistics programs available online. There is no substitute for knowing what your particular load is doing at all feasible shot ranges.