Sentences get a zero when they have multiple errors, or errors so devastating that I can't sort them out. I add up the sentence scores and divide by the number of sentences. If the quotient is less than 1.5, I have a paper in which more than half the sentences are not proper sentences. That by my standards is a paper I should not have to read. If that seems too high a standard, check it against your practice.
Different instructors have different tolerances for bad sentences.
That's better for the student, for whom unfavorable commentary from beginning to end is more depressing than instructive. And anyway, I am usually willing to allow the writer to have another go.