Others have offered up a sighting of roughly 2 inches high at 100 yards as a good sighting scheme. In my own experience I have come to favor a sighting of 3.5 inches high at 100 yards. This allows for the individual to hold dead-on (directly in the middle of the top and bottom) the animal out to roughly 350 yards.
Magnum calibers such as the 7mm Remington and 300 Winchester will extend this slightly. At 400 yards I hold directly on the backbone of the animal. The drop at this range allows the bullet to strike within the thoracic cavity. At 500 yards (my personal cutoff under no wind conditions) I take the height of the animal's brisket and stack that on top of the backbone and use this spot as my hold. Lucky for me, the duplex post in my scope corresponds with this hold if I have the magnification set to 9X.
Use this sighting method to eliminate confusing holdover.