> 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.