peoples fingers vary, but as a rough guide you shouldn't be able to get your little finger into the gap at any point. On mine, at its worst, I could get my little finger in beyond the nail - can't get it in at all now it's been sorted.
Gap should be parallel etc etc...
Chrome should also line up without any gaps too ('belt line') at A panel like Paul says. Mine was only a mm or so out here (again, now sorted), although I've seen some absolute horrors that had a misalignment of about a cm!
Some people describe the problem as "it's as if the bonnet is still open". It's worth pointing out that in some cases of the problem the bonnet may need an extra push on both sides to make sure it really IS shut!!
SSL
(a Fender Precision owner, funnily enough)