I was thinking of determining the percentage of battery with a voltage divider bridge, I had calculate a resistance R1 of 27k Ohm and a resistor R2 of 10k Ohm then I wanted to make a cross product to calculate the percentage with the voltage determine at the output of my voltage divider bridge.
But I'm not sure how to do it.
Start with the Arduino example sketch for analog input. It reads the voltage on a potentiometer using an analog input pin and flashes a LED. You can expand on that program to do what you want.
Replace the pot with your voltage divider and you battery. Check the voltage to ground with your DVM, meter. must be less than 5 volts.
Be sure the battery and voltage divider are connected to the Arduino ground.