Ok, here's what I tell my students (I think it's the Larry Teal method but can't quite remember where I read it all those years ago):
1. Take a piece of paper and slide it between reed and mouthpiece.
2. At the point where the paper meets firm resistance (the meeting point between reed and mouthpiece), make a pencil mark on the bottom of the reed.
3. Place your thumb on the reed with your nail facing back towards you and positioned perfectly on the line that you drew.
4. Put your bottom lip there.
While I realise that I tend to move about quite a lot on the piece for different tones/effects etc, I always find this to be a good starting point and a clear way to explain it to students. It makes sense logically aswell.