Hi Phil,
The distance is measured from the target face to the rear of the line (or barrier) that your toes must be behind.
The rifle or pistol that you are shooting may well extend past this point, but as long as your toes are behind the shooting line that is OK (and is usually the case).
If you are shooting behind a bench or table etc, no part of your body or clothing must be able to touch it as it could be construed as giving you some extra (illegal) support.
Simple really, and easy to set up at 10 meters (or 6 yards - the original British target distance for air weapon target shooting).