You don't need millimetric accuracy. If you are within a foot or so of a line 200 yards long, that would be sufficient. We could test what a non-dowsing person is capable of under the same conditions with anyone who is prepared to give it a go. The comparision of results would be interesting. If you can exceed the best of the rest by a good margin we'll enter the Randi contest and split the winnings!!! :o)