The charge courve of the battery type is very flat.
LiFePO4 Voltage Chart (3.2V, 12V, 24V & 48V) - BatteryFinds
Almost all the charge is between 3.3 and 3.2 volts.
So I would need an accuracy of about 0.05 volts I think. Especially if the battery drops from 3.30 to 3.25, the reference voltage needs to be steady enough that although the input voltage of the ESP varies in that range, the reference voltage needs to be steady enough that the ADC measures that difference.
I have no problem with manually adjustig/calibrating the voltage values. It is just important for me that I can still measure wether the state of charge is, lets say, 60% or 40%. I dont need an accuracy of 1% of state of charge, but at least 10% would be good