[mrpopo]

Again, being able to live a life of leisure exclusively out of your fortune's interests reaches far higher than "living a comfortable line". You can live very comfortably with a part-time job, or even a full-time job that's not too demanding.

[rgbrenner]

The question says "without thinking about finances again". It is true the number could be lower, but there does need to be some buffer here. Otherwise you're one mistake/accident/recession/layoff away from "thinking about finances again".