I think you got the right idea cnote, slightly down like during a nod is what you want. You can hold an orange with your chin, and your chin will be right where it needs to be.

Scrap also mentioned in another post that it effects your breathing (which in turn affects your eyesight) if you have your chin down more that 15 degrees. Have you ever read a book in bed and put your chin onto your breast bone? It's uncomfortable and makes it harder to breath, think about bending a garden hose, less water gets through. You don't want to restrict your airways.