...but the problem I've always had with these types of systems is that there's always that one item on your to-do list which ends up more complicated than you expected: So at the end of the day you're stuck with either (1) redefining the scope of that one to-do list item so that you can enjoy your evening and browse the web or (2) sticking with the original scope of your todo item and then you don't get to enjoy your websites/evening, even if you've worked hard otherwise.
In this way, what was supposed to be a cut-and-dry, objective productivity metric unavoidably devolves into a subjective decision.