Determine a known battery's instantaneous capacity through voltage? / voltage's relation to capacity
There is memory or hysteresis in voltage due to a double layer effect.  There is also slight variations in polymer chemistry in particular Fe or iron that affect the relationship.

AFAIK 3.7 is 90% and 3.0 is 10% as a resting voltage.

ESR affects voltage drop or rise due current x ESR and this rises faster as SoC drops near and below 10%.

Peukert’s Law applies to battery efficiency, partly due to memory effects on ESR.

The 0.5C is the best trade off for speed and storage efficiency yet 1C is often used still others will use up a battery in 30 minutes or 40C.


So try my estimate and confirm with your datasheet.
Graph 3.0 to 3.7V as 10% to 90%
Then use the recommended CC, CV and %CC cutoff value to determine new capacity from datasheet. E.g. 100% @ 1C will take longer than 1h on new cells but less time on aged cells as the capacity is reduced.  Keep records and serialize each cells charge and discharge times. Learn from your experience with use and brands.