[Vwdiesel] Anyone know how to use the VW210 TB tension gauge?

[Kneale Brownson]

Anybody have step-by-step directions for using the diesel timing belt tension gauge VW210?  You can see one at  http://www.pbase.com/kneale_brownson/image/84722362 
   
  I understand that the belt goes between the fingers on the base of the tool and the spring-loaded plunger that sticks out of the barrel with the numbers on it.
   
  The Bentley says I need a measurement of "12-13 index marks". How do I achieve that measurement? Where do I start? Are "index marks" the white numbers on the barrel or the black numbers on the handle?