Just read about how to do a loop in a real plane and you'll find the answer to this question. A perfect loop results in the plane coming out of the loop at the same altitude that you went into the loop. If I am remembering corectly, real pilots in real planes don't totally kill the throttle... they lower it to about 1/3. They don't do this at the top of the loop, they do it as they begin to come down out of the loop, or at about 2 O'Clock if you imagine the loop-circle as a clock. You then go back to full throttle as you level out. If you do it exactly right you come out of the loop at almost exactly the same altitude that you entered it.

Of course, this is all from 15-year-old memory of playing computer flight sims, so I might be forgetting a step or two:-)