Thanks!
OK, here's what happened. The Hakko soldering station FX-888 I bought was NOT a fake, so I'm making a rash assumption that it's calibrated since I just got it. If it had been two years, then no ...
I set up the station and FG-100 to make a measurement. The FG-100 senses room temperature and that was spot-on. I turned on the FX-888 and set it to 300C. I waited a little while and put a small spot of solder on the tip, and touched it to the sensor.
I kid you not, I am not joking, and I was shocked -
The FG-100 read ...
300C
It was not even off by 1 deg at first, then went up to 302C after a bit. 