Interesting solution - the zener will have an almost constant current across it, so the voltage will be very stable and the resolution is going to be great, because you utilize a large part of the range of your ADC. I kind of like it! This being said, the chemistery of the batteries is complicated and the voltage will fall under load, then it will slowly recover under no load, etc... I think the best way to do this is coloumb counting, where you measure the current drawn and then calculate the used up capacity. There are special opamps (INA180) or even current sensors (INA219) that you can use to convert a value across a shunt resistor to something useful.
I'll be measuring battery capacity by counting amp hours since I know the exact total capacity of the battery (31Ah). I looked at INA devices and they are only capable of 3 or so Amps and I need something up to 60A. That's why I'm thinking of using ACS758 current sensor. I would be very thankful for a schematic or diagram of the zener diode solution. I don't want to mess anything since working with lithium-ion batteries is quite dangerous.
Sorry for my english, I hope you understand what I'm trying to tell.