My uncle is a psychiatrist, and he has a theory that the definition is more of a matrix than a spectrum. He says that what is often called attention deficit would better be described as intention surplus - you have more intention than your attention can handle. A lot of A.D.D. cases are confused by the fact that they can spend several hours hyperfocused on something, so it's not a lack of attention that's the problem, it's that there is a limited supply of attention which cannot keep up with one's attention.

Putting it this way opened my eyes to my own diagnosis (diagnosed A.D.D as a child).