Personally, I think this method of goal-setting is terrible and in many ways, it's a way to set yourself up for failure. It works well if you're doing NaNoWriMo and need to reach 50,000 words by the end of the month, but outside of that, it's a very inefficient way to write. It forces you to focus strictly on quantity and ignores so many other important areas of writing, such as editing/revising, outlining, querying, and marketing.
With that said, setting a written goal, not only in writing, but in everything you do, is vital if you plan to achieve an ounce of success. I just go about it a different way. We're busy people (humans), and the not every day is the same, therefore you can't have a one-size-fits-all writing goal. Your writing goals also must included the other areas of "writing."
If you have a job and a family, like most aspiring writers, you'll have to figure out a way to fit writing into your daily schedule and, more often than not, your "real life" will get in the way.
What you need to do is set a daily goal based on your daily schedule. I start the day with a long commute. During that commute, I come up with a priority list of 1 to 3 writing goals for the day.
Here's an example of my writing goals last Friday: 1) Critique a story or two on Scribophile. 2) Read over and submit a story to Scibophile for critique. 3) Write a 500 word flash story.
And here's last Saturday: 1) Revise story based on the Scribophile feedback and submit to publisher. 2) Read over a chapter from the novel and continue building the synopsis for agent queries. 3) Write a blog post, time permitting.
Any of the "success" authors and speakers out there will tell you that if you break down your big goal into smaller, more attainable goals, you'll be more likely to keep pushing along until you achieve your big goal, whatever that big goal is.