There's nothing wrong with Rule 18 in theory, except that it rewards the liars. No way way to prove you are inside or outside the zone.
You can often "out" the liars if you have several people with good faith also recounting the incident. THe rare occasions I've chaired a protest hearing, I keep them seperate so they can't hear each other's tales. For the zone, I'll get a racing mark and set it on the lawn, and ask them to walk to a spot where their boat was, relative to the mark. Then measure it. Don't measure it first, that produces bias.
3 boat lengths is a lot further away than most people think it is, from what I have seen.
But getting back to liars... I've had hearings where the less-honest skipper was changing his story as he heard the salient points of the other skipper's recounting of the incident. That's a dead give away.