I think your plan to use biorhythms to plan how much work you can comfortably do in a day will succeed. Most of the time the biorhythm calculator agrees with how I feel. One time it didn't, but that's only one time. And there are other factors to consider - working with a friend, doing something I want to do vs something I don't want to do, illness, bad things happening to me, and so on.
Coincidently, I've been wondering about the same thing. I started by setting a goal for myself of working 5 hrs. each day. The next week I increased it to 5.5 hrs. and so on. I'm up to 8 hrs. I find that five to six hours is a comfortable goal.