in addition to the curve you'll extrapolate to estimate the remaining charging time (e.g. starts charging with 35% @ 4040mAh equals 40 minutes remaining according to the charging curve) maybe you can collect data on the phone each time it's being charged, so that you'll generate a curve specific to the phone.

So you'll know that for this specific phone, 4040mAh equals 32 minutes remaining for this phone, instead of 40, because the last 10 chargings, on average, there was 32m remaining @ 4040mAh, 25m @ 4100, 15 @ 4150, 5 @ 4170.

And because a battery degrade in time, and a new firmware can also improve - or not - battery duration, your program will always be accurate.

I'm maybe wrong, but it's how I see the thing