I don't think an automatic rule like a percentage is necessary. In some cases, there may be legit DNFs (injury, sickness, family emergency, weather) mixed in with ones that were more suspect. However, if players knew that TDs might report them to the PDGA for disciplinary action for a pattern of DNF behavior, that may become a deterent. Like I said, some players have been contacted and some disciplined for DNFs especially when connected with bad behavior.
Rater of the tossed arc.