Being the entirety of not is not always relevant to fair use. Of course a judge may take amount into account, especially when arguing damages, but fair use is a guideline to a judge not a set of well-defined rules (though collection societies love to print their own policies as being the rules...)
> fair use is a guideline to a judge not a set of well-defined rules
There are four factors of fair use. Factor three is the amount or substantiality is being copied. You're somewhat right there isn't an entirely objective standard to measure things, but there isn't exactly an objective measure to creativity.
You can't put something on a scale and get units of creativity a work has. You can't get a graduated stick and measure the creativity of a work.