If your vehicle has an adjustable headrest, how high should it be?

• So that the back of your head is at the center of the headrest
  Explanation:

  "If your vehicle has an adjustable headrest, it should be at the right height. The back of your head should be directly in front of the middle of the headrest to protect you in a collision."
• So that the back of your head is above the headrest
• So that the back of your head is below the headrest
• There is no such use of a headrest