The starting point of the eyebrow should be straight above the center of each nostril. You can avoid making a mistake by marking this spot. Align a ruler vertically from the center of each nostril. Use an eyebrow pencil to mark the spot at the proper eyebrow level.
Then position the ruler diagonally from the edge of the nostril to the outer corner of the eye. Mark where it crosses the brow, this is the end point of the perfect eyebrow. Tweeze or fill in the hair with a pencil to make up the difference.