Unfortunately nothing is set in stone with poster grading, unlike the comic, baseball card, stamp, kind of collecting. So you just have to judge for your self.
Personally I had mine on a 1-10 scale using half's if necessary. 1 being totally knackered, and 10 being mint, but that was only for me personally so I knew
what condition the copy I had was in.
You will find condition descriptions a right pain, just go with how it looks to yourself.