RE: flying behind the flightline
What works for one group may or may not be safe for another. As an example, sailplanes have different needs in clubs devoted to sailplanes than 42% gas planes would at a club for powered flight. You would probably not need 400' at a park flying site. It is up to the individual or club to come up with rules that make sense, and keep the site safe.
This is the actual Safety Code Rule (RC #3): At all flying sites a straight or curved flightline must be established, in front of which all flying takes place. Only personnel associated with flying the model aircraft are allowed at or in front of the flightline. In the case of airshows, demonstrations, or competitions, straight lines must be established. An area away from the flightline must be maintained for spectators. Intentional flying behind the flightline is prohibited.