In my opinion, the FuelGauge is pretty accurate.
Several assumptions are made, and you would have to make the same assumptions to calculate a SoC independently.
I think the misconception comes from the firmware’s default ChargeVoltage at 4.11V for various safety reasons.
The SoC shouldn’t be mapped or re-calculated just because 4.11V is the Vmax selected for charging.
The SoC does a good job as it uses the known Li-On characteristics to approximate the available chemical energy at any give time. But 100% SoC will always require a terminating voltage near 4.21 for 3.7V Nominal Li-On types. We have the option to set the ChargeVoltage to 4.208V.
Most Li-Po’s are rated by calculating the mWh from discharing from ~4.22V down to 3.2V or sometimes 3.0V (per the individual datasheets). The mAh rating is calculated using the Defined Nominal Voltage (usually 3.7V) and the total mWh during the discharge test.
So take the 2,000 mAh (7.4Wh) Li-Po that we all use for instance, we will never realistically use all 2,000 mAh because we’re not going to discharge to 3.0V. We can consider at-least 20% as “reserve” capacity if ya want.
But re-calculating a different SoC (if you stick with the Default 4.11V Charge) will be much less accurate than just using the FuelGauge.
For instance: if the FuelGauge reports 85% SoC, then that Li-po has approximately 85% of the maximum chemical energy remaining (if you discharged it all the way down to 3.0V). That % is basically independent of the mAh rating of the battery or the ChargeVoltage used.
A simplified calculation:
85% SoC reported for our 2,000 mAh Li-po = 1,700 mAh remaining
But we also would subtract the 20% “reserve” that we never want to reach.
So instead of using the reported 85% Soc, reduce the SoC by 20%.
65% SoC of 2,000 = 1,300 mAh
(SoC - Reserve) * Battery mAh Rating = usable mAh remaining.
That will give you more meaningful data than calculating your own SoC from the Li-Po Voltage, in my humble opinion.
Again, the FuelGauge isn’t inaccurate. It actually does a great job.