I'm not sure I buy your ability to understand your own productivity (people are notoriously bad at that). It's hard to believe that more work doesn't strongly (but certainly imperfectly!) correlate with more output... Up until that work exhausts you to the point and you start doing BAD work. Maybe that # is 35h/week. I suspect it's widely varied based on type of work, type of person, depth of love for the work, diet, exercise, etc.