For me, it's not based on the work, it's based on the mood/mind. A "good day" is one where I was thinking clearly and solving problems and choosing good paths with ease.
In fact, the amount of code written can often be inversely proportional to the quality of your work in that day.
There simply is no meaningful correlation. It usually takes the talent of another skilled professional to measure the quality of your work, which is one of many reasons why management exists.
The amount of code written doesn't mean as much.