Bore Sight in at 25 yards, then, take Your first shot, holding onto the bull, now, after the shot, hold rifle on the bull and adjust scope so it aligns with that bullit hole, now, take Your next shot, if it is`nt on the bull, readjust as in step one.
When satisfied with those results, move out to fifty yards doing a three shot group, holding on the bull, group will be above the bull, do not be concerned, leave it above the bull, adjust scope so it is grouping tight but about two inches above the bull.
Now it should be about dead on at 100 yards.
Same procedure at 100, when satisfies with those results, if You are going to be using 180 gr. booolits, then, move the cross hairs so it is shooting about four foot above the bull.
Move out to a 800 yard targut, use a 4X8 sheet of plywood and one hell of a powerfull spotting scope or a telescope might work even better or a spotter, with a spotting scope set up, out of harms way to radio the results back to You.
Now, start the procedure as stated above until You are satisfied with how the booolits is grouping.
Try it now with a deer size targut, practice, several hundred boxes of boolits and shooting in all sorts of conditions and Your chances at that 800 yard whitetail`ll greatly improve.