That would probably work. That's how I was originally planning on doing it, but I got annoyed with the voltage logging not working properly when I set it up so I gave up and just went with the linear voltage -> capacity reading. A potential problem with this is that every battery pack would be different, and people who make this project on their own would need to generate a table for their pack.

Not as simple. Voltage also changes with load and temperature for multiple reasons. You'd also need to account for that.