Don't know whether that makes sense, or not?
My question was based on the belief that there is a specified size and therefore related to that size on the underside of the wing.
The reason I ask is that yesterday as 4 aircraft flew over in very loose formation I naturally looked up out of curiosity and was surprised at how easily I could read the reg. of the one which went directly overhead. It never crossed my mind that they were particularly low so I was surprised at being able to read it (not that it would have mattered too much as it was over the sea).
They seemed to be enjoying themselves.
BB