I wish the new expansion board would have a battery management chip on an IIC line
not sure if you have seen this.
To summarize, what you are trying to do is create a analog sense of a voltage divider circuit.
The exact reading of the voltage directly is not possable. You have to map the ADC to a calculated voltage based on calibration by trial and error. This requires an understanding of how ADC work… Voltage Reference line and This is why it is difficult to answer your question… HTH