is the right answer.
It could be that "up to 5 ticks" that the above poster mentioned seeing on a 9.0, could also get you an 8.5, or some other grade, on a different book..
I wish there was a definitive answer to this too. Sorry, but I don't think there is, at least certainly not from CGC.
Perhaps some seasoned veterans (collectors/dealers) out there have established their own methods though, I'd be interested to hear about those.
For me anyway, any more than 3 color-breaking spine ticks or dings eliminates it from any sort of 'NM' designation instantly, unless the rest of the book is pretty much flawless. Not sure if that makes me picky, but that's my thought.