Hi Patrick!! Big fan!! cough
I am indeed very much aware I am running on unreliable hardware. My evaluation of my productivity comes from reviewing things done in micro and macro scale, as well as feedback from people around me. Seeing my HN history from PhD years to Startup years is night and day. I've even installed RescueTime to get a better idea of how I spend my time. I try to get as much external data as possible, always happy to apply any other (sensible) metric you (or others) can suggest.
But keep in mind these same considerations hold for everyone claiming they're super-productive in 40 hours a week.