Back then when ED implemented the PFM for the F-15 I made a 90+ mins roundtrip at different speeds and altitudes and came home within 300 pounds of what I´ve calculated (which might be down to my sloppy flying).
I know they made some adjustments to the performance and power output after that and I have never tested again, but at least at one time, and for the F-15, it was pretty much spot on.