Crystal Oscillator Accuracy

I thought you said that there was a tiny amount of non-volatile memory outside the normal flash on the STM32 chips?

Sadly it's only nonvolatile if there's a battery connected - there's 64-80 bytes that survive a reset, but when power is disconnected it's gone.

maybe could it be just after the bootloader in flash page 0

Yes, for writing once that's definitely possible. It then gets dangerous trying to change the value though.

Nothing stops the value being written alongside the saved code though... I think that would probably still be fine? I could make sure reset() didn't clear the calibration data and then you'd be sorted.