This raises another question - how do you ensure fairness between co-founders. P...

This raises another question - how do you ensure fairness between co-founders. Particularly if one comes into the game later, or takes an extended break, or simply can't contribute as much as the others (e.g. has family commitments).

Is this usually settled in the division of equity? If so, how hard is this to change over time? I'm sure once you're rolling this wouldn't be overly simple.