Ratio Between Voltage and Charge
Battery Voltage mostly depends on


State of charge
Current being drawn, because of internal resistance.
Temperature
Other aspects of its physical health, age, etc.


None of these are simple or linear.


Voltage vs State of charge, you will find a chart for your chosen battery chemistry. It may have a nice slope (Lead acid) or it may be very flat for most of the discharge (lithium, NiCd), in which case the voltage gives very little useful information.
Internal resistance should be specified for the actual battery. It also changes with the state of charge, you might find a chart of this. It might not be a linear resistor, and it does depend on the frequency of the current drawn.
Temperature, will probably have a simple relationship, also in the datasheet.


Simple battery indicators will measure voltage, perhaps compensating for temperature.

Advances fuel gauges, like in your cellphone or laptop, actually count the charge in and out of the battery. They compare this cycle to a known previous cycle when the battery did actually run down. They take into account the charging efficiency, temperature and aging of the battery too.

The best advice I can give is to read the whole of batteryuniversity.com. He knows simply everything about batteries and has taken some trouble to write the book about them. He even blogs about them, like when Boeing was in the news...
Also read some manufacturers' datasheets for the chemistry of your choice, to get an idea of the characteristics you can expect.