Just your basic scribe tool.  Designed and proportioned for your tool bag.  Raised numbers are for better visibility.  Pencil holder is sized for a standard #2 octagonal pencil.  A relief cut out is provided on the bottom to aid getting the pencil tip as close as possible to the scribe surface.  

Rotate the fraction of the (amount that you desire to scribe off) against the mating surface--push the pencil tip down just till it touches the scribing surface and scribe away.