ADC calibration in ATTiny88
The internal band-gap reference is not very accurate - it is just stable (i.e. won't change much with temperature and over the life of the chip). So you will need to calibrate using a known voltage and store this into the device as you described.

Since the ADC has good linearity by design you can use pretty much any voltage for working out the scaling factor as long as you know what it is.

Another option, though more expensive, is to fit an external band-gap reference to the circuit and use this as the reference for the ADC.