I did FF yesterday using my pacing plan based on my Xert model with mixed results.
The plan worked well enough for the NM/MAP part of the test reaching MPA = Power at the end of the MAP segment. I was almost at MPA when the MAP segment started. The NM estimate was pretty close, the MAP was overestimated.
The FTP pacing did not work out. There seems to be a relationship between MAP and 20 minute/FTP power that Xert does not capture. I was exhausted after the FTP segment, but the power level was nowhere near MPA.
The Xert estimate also did not come close to MPA = Power at the end of the AC phase, even though I had a 25% improvement there on the test, and my stomach was telling me to stop. I ignored it.