Is 'a bit of fat' an esoteric engineering unit of measurement, JT? Sounds to me like a colloquialism for 'margin'...
Whatever it is, it's built into the number.
Given that I'm 'on my own' because the scenario I brought up is outside the certification parameters, my decision to unmake the rejection decision on discovery that my brakes have failed will depend on the spool up time for the good engine/s that have been spooling down for only a few seconds, noting that I'm over V1...