A variable is no different than a fixed power scope. Assuming you had it bore sighted when you had it mounted (If not, and its not a bolt action, you better start real close to your target - maybe 15 yds., just to make sure you are on the paper.)
Once you get it close to hitting where you want, and this is where a friend comes in handy. Take a shot off a rest, then while on the rest, aim your rifle at the bull, hold it still, and have a friend move the cross hairs to the previous bullets point of impact. If you didn't move the rifle, your next shoot should be in the bull, or very close to it.