Suggestion to FA folks:
This subject has come up many, many times, as you know. I don’t think that FA is calculating “wrong”, but it does seem to confuse the heck out of folks when they compare times with another system.
How about considering adding a way to list BOTH ETAs: “Planned Arrival” time based on the pilot’s flight plan, and “Estimated Arrival” based on the current speed/distance calculation.
Incidentally, I think you are actually doing it the “right” way now if you are only going to list one ETA. If the pilot says he will get there at a certain time, that is probably the time you should post. You don’t know whether he’s going directly to the airport at the current speed or or has some other agenda in mind, so should “trust” his estimate. However, we have seen that many of the estimates are pretty far off, either from sloppy planning or unexpected conditions. An FBO using FA to plan for an inbound will likely want to know the earliest time he might show up, considering current conditions, so they can be ready to provide excellent service. Give them both ETAs and then who can complain? Somebody will, of course, but that’s another issue!