Author here :) I'd echo this. It's really crucial that you limit your goal for the day to something that you have nearly 100 percent confidence about. So even if you think "I can do this whole thing today!", you should just pick off the first piece of it that's definitely attainable to use as your goal.
In addition, on the few occasions where I didn't manage to hit my goal by the end of the day (maybe three times in the last month), I'd take the time to figure out why that was and think of how I could have reframed the goal in a way that would have made it achievable. Usually when I missed a goal it was because it was either not specific enough or it was dependent on something else. For example, I might have had "Push XYZ patch", when it should have been "Make bugfixes to XYZ patch and push if able", since there's always the possibility that the latest fixes introduce an unexpected problem or that the reviewer won't get to the code review that day.
In addition, on the few occasions where I didn't manage to hit my goal by the end of the day (maybe three times in the last month), I'd take the time to figure out why that was and think of how I could have reframed the goal in a way that would have made it achievable. Usually when I missed a goal it was because it was either not specific enough or it was dependent on something else. For example, I might have had "Push XYZ patch", when it should have been "Make bugfixes to XYZ patch and push if able", since there's always the possibility that the latest fixes introduce an unexpected problem or that the reviewer won't get to the code review that day.