While I had the case open, I checked the FG-801’s calibration:
Look at all those parts!
The raw filtered DC power supplies run a bit high and the output voltages & frequencies were off by a little, but not too much after all these years.
Page 11 of the instruction manual gives the setup and calibration adjustments (clicky for more dots):
Page 12 gives some values that should be true:
For whatever reason, the manual isn’t available on The InterWebs, so here it is for your amusement: