It's mathematically accurate but not reality. What is that you want to do?
To know where you are on the "Fuel Gauge" you have to know the usable voltage range.
If we know the max and min voltage we can calculate where we are on the "fuel gauge"
% full = [1-(Vmax-Vmea)/(Vmax-Vmin)]*100
If we assume it's 4.2 -> 3.2V then it look like this.
However, to
@Greywolf74 point 3.2 is a bit optimistic. If you know what your LVC is then use that as Vmin. Let's say 3.5 is a good number to start with and if your measured voltage is 3.7, then you are at 28% capacity.
%full = [1-(4.2-3.7)/(4.2-3.5)]*100 = 28.5%
But wait, it's a bit more complex because the voltage is not constant. It changes a little bit with temperature, load and charge state. It's not uncommon to set your LVC for 3.5 and get 3.7 when you check it later because the voltage will droop under load and it regains a bit of charge after resting.
They also don't discharge linearly. This is from the Battery University above, it's 3V cells but it's similar for LiPos. Most of the time you don't want to be on the tailing end that falls off the end of the earth. The curve demonstrates the usable life of a LiFePO4 is much better than a A123. The curve is flatter longer.
The actual curve depends on the battery and configuration but most manufacturers don't publish the curve, and if they did it would only be valid for a short period of time, because it degrades with use.
