Ask HN: When to clock in, when to clock out?

I am in the wonderful position of setting my own hours. My boss likes me to work a certain minimum amount (because, so he says, his job is to make me rich), but the main concern is that the work gets done and when I do it is completely up to me. However, I am often puzzled over just what and how much time I should count as "doing work", particularly when so much of creative professions like programming is just thinking about the problem. At one extreme, I'd only count the time that I'm actually typing or in meetings; at the other, I'd say "I'm on the clock if I'm even thinking about a client." Obviously, neither one is a very good idea, so how do you know where to draw the line?