Your natural gaze is actually not horizontal, but 10-15 degrees below the horizontal plane. Check this yourself by closing your eyes for a few seconds and opening them slowly. You want your natural gaze to fall on the middle of the computer screen so that your eyes will not be strained and your head will not tilt up or down. Use a monitor riser or a few books to prop up your monitor if needed. Get a co-worker to help you out with positioning if you have difficulty.