If you're getting good bare shaft flight, that's your answer. Most folks use a higher nock point for 3 under than 1/2". I nock the arrow under the nock point, and my bows tend to need a nocking point between 5/8" and 3/4", depending on the results of bare shaft testing.
If you find your arrows hitting higher than you're looking, feel free to move the nock point up a little, as that will cause your point of impact to move down. You generally have more room to move the nock point up than down before you get erratic arrow flight.