Depends on the elk. What kind of scope are you using? If it's something like a Leupold Duplex, you can look up the height of the thin center hairs and calculate that value at various distances. For example, the thin lines on my 2-7x33 are 8 inches high/wide at 100 yards, which means the total opening is 24 inches at 300 yards. So, to account for a 6 inch drop at 300, I center the target between the cross hair intersection and the point where the wires go from thin to thick. It's not perfect, but it works for me.
Where do you want to hit?
Top of back would be a touch low for a high shoulder but too high for the good stuff. I would figure back to brisket about 24" on a mature Bull. Also, the hump is from the "combs" coming off the spine, not the spine itself. I dial so I don't have to think about it.