Thanks. I suppose that could "work" but think it would be quite crude. Of course, does not work (well) if you're using the phone while on the charger. Plus, different batteries, chemistries, and varying degrees of battery health probably means that "cut-off charge current"varies all over the place.
The convenient thing about battery management systems, is that, most good ones, actively monitor the charge/discharge characteristics (impedance) to analyze the health of the battery pack. So, the reported state of charge is relatively accurate, and there's no need for "calibration". By calibration I mean: If I was just measuring input current, then the current at which 80 or 90% SoC is reached will likely be different depending on factors I mentioned above. But, when a BMS reports 80% SoC, you can believe that the battery really is charged to 80% +/- 5%, or so. No need to do any analog measurements.
Thanks for the idea though. Maybe there's still a way.