I've always looked at the 1-10 scale as percentage points.
Anything under a 6 is a "fail", not even worth f**king.
A "6" is a "D", agirl you'd f**k, but wouldn't want anyone to know about it.
A "7" is a "C", a girl you'd f**k and not mind if your friends found out.
An "8" is a "B", a girl you'd f**k and WANT your friends to know about it.
A "9" is an "A"...these are girls that everyone drools over. True "9s" are rare. They get snatched up quick or are in such demand they are hard to hang onto.
A "10" doesn't exist.
The 1-10 scale can ONLY be used to judge looks. It is no good for personality, because everyone is looking for something different. But generally, society seems to agree on what makes a girl "hot" in a given culture...what generates sexual attraction, barring fetishes.
Here's the thing...girls aren't stuck to one point or another on the "scale". There are 6s who can easily get to 8 or even 9 just by making a conscious effort to lose weight. There are girls who are legit 9s, but look like 7s when they dress down. I ran into one of these out at a college bar one time...it didn't really hit me until Date #2, when she REALLY dolled herself up, JUST how good-looking she was. (which is probably why it was an easy pull...no anxiety) When girls get their hair and make-up done for a wedding or school dance, that can instantly knock any girl up 1 point on the scale.