Interesting question. I have always worked to the 2.5nm quoted by Reynolds No1 as far as the question "Did he exceed the cleared area?" goes, but apply a "Pilot Performance Standard", which is arbitrary and based on what the average pilot can do on our (helo) type. That limit is 0.5nm, which was also the limit used on IRTs when I did them in the RAF. Absolutely no documentary backup for that one, so shoot at will...but it does improve the standards.