Mutt,
Sorry mate, I was just querying what CC was actually saying.
I don't think that there is any doubt about how the Fixed Reserve is expected to be calculated - there should be a stated holding configuration and there should be matching fuel consumption data, hence the quanta is easily determined. Generally, this is manufacturer's data but can be properly derived operator data - it is not normally dictated by the regulator. I couldn't fathom who CC's "they" was.
I could not (and still cannot) see how cruise fuel flows would get a look in for calculating FR.