And here it is your answer: the tip should touch the top of your belt. Have a look at the picture. The tip should never go longer than this. It could be a little shorter but never to the point of showing some shirt between the tie and the belt.
If you are not wearing any belt, just imagine where the belt would go or just don’t leave any shirt showing between the tie and the pants. Got it?
And if by any chance you want to break this rule, go shorter (but just a little bit, showing an inch or two of the shirt), never longer. Deal?