Anyway, never ID a buteo based on this feature solely, and NEVER EVER identify the race of a Red-tail based on this feature alone…you will be wrong as often as right, and it will have been luck if you are correct anyway!!! Besides, there are way more reliable field marks to use when telling perched birds apart. I'm not saying to never use this trait, its just the very last thing you should look at. I know wing tip to tail tip ratio has been verbalized a million times over the years, but that doesn't mean you shouldn't question it. In fact, question everything you hear or read when it comes to bird ID, because I can tell you with certainty that there are a ton of errors in the literature, and inaccuracies perpetuated out loud by the every-day birder. I tell people all the time, regardless of what I say about raptors, research it for yourself to really understand it or see if its true.
Check out these birds just to note their posture and where the wing tips fall in relation to the tail tip. Especially on the first 3 birds, which are Red-tailed Hawks