Depends
When I'm drafting, I try to get in a set amount of words a day (Though I'll settle for getting a tricky scene right)
When I'm editing, I go by other criteria. Pages done, for copy edits. How many knotty problems if content editing
The important thing is to have a goal and try to reach it. This means you are making progress (if if you fail in your goal today, you have made progress)
Not having goals (for me anyway) means I faff and achieve nothing. If I reach the goal GREAT! If I miss by some, well at least I made things better than they were yesterday. And I can catch up tomorrow when I hit an easier patch
Have a goal, something to aim for. Long term and short term (Today I want to write X words, and/or finish this draft by Y)
If you do not meet your daily goals because life* then don't throw in the towel. As long as you are meeting your longer term goals (or close), you're good
*I have a dodgy disc in my neck. Sometimes it plays silly buggers and I have had the neck/headache from hell today, plus a lot of work related stuff -- redundancies -- oh and the train got cancelled this morning and I cannot write on the replacement bus because no room for laptop. Today was a bust pretty much. But tomorrow....