What is the correct method of calibrating the zoom lens to keep it in focus throughout the zoom? ...

What is the correct method of calibrating the zoom lens to keep it in focus throughout the zoom?
 
  a. zooming the lens all the way out and focusing on the background before zooming back in again
  b. zooming the lens all the way out and focusing on the principal subject before zooming back in again
  c. zooming the lens to the normal focal length and focusing on the principal subject
  d. zooming the lens all the way in and focusing on the test chart before zooming back out again
  e. zooming the lens all the way in and focusing on the principal subject before zooming back out again