look in the USASA rule book. the distance is between gates; i.e. if you take a rope at 20m, and tie it around the stubbie of the gate above it, you can put the next gate at the other end of the rope. the horizontal distance will depend on the slop of the hill. steeper sections will be set more across the hill, to control speed, and flatter sections will be straighter. best thing to do is run your courses and make sure they feel right, compared to courses that are set by more experienced setters...
-Noah