So I think we all know that it's not smart to call a beautiful girl, beautiful, as she already knows that and most likely hears that on a daily basis. Thus that compliment no longer holds any value, and you come across as an unoriginal.
But at what point does the prettiness of the girl surpass the threshold, in your opinion, of being "too" pretty to be called beautiful, to the point that the compliment still holds value?
Based on math, the uglier the girl, the more value that word holds, but as the prettiness goes up, you get diminishing returns per investment.
The math would dictate that 10 would be too pretty to use that word, but I think the actual threshold is subjective and would differ per person, because one might feel like "it's not worth it anymore after a certain point, because even though you get "some" return on investment, it's not a significant amount".