If you use a start and finish point, the Start and Finish lines are 1000m long (500m radius). Furthermore, you need to cross them in the correct direction. If you use a 500m CYLINDER observation zone at a turn-point, and turn just short of the Turn-Point, 1 km distance gets deducted. However, if you managed to go around the turn-point, you get scored full distance as the Sector observation zone will then be used.