For "average" .308 Winchester, 150 grain (for deer, right), just sight it in 1.25" low at 25 yards. You'll be deer-accurate all the way to 280 yards. That equates to sighting in dead on at 240 yards, or 2" high at 100 yards.
Like (the wise) OldOwl said, point-blank range is a good way to sight...