I don't rate girls by numbers, I just associate them with words like cute, sexy, fine, beautiful, etc.
But if I had to make a scale for me it's like this:
HB1 - Unbearably hideous
HB2 - Very, very hideous
HB3 - Ugly
HB4 - Below Average (D)
HB4.5 - Below Average (D+)
HB5 - Completely Average...