Proofs are graded in the following way. (This goes for proofs on exams, too.)
Any complete and correct proof gets full credit. It doesn’t matter how many steps you had, whether you used the most clever strategy, etc. If your proof works, you get full credit.
If your proof is incorrect or incomplete, here is what I do to calculate partial credit:
I begin by deleting any unnecessary or incorrect steps you contributed. There are two qualifications on this:
I then correct or complete the proof in the fewest possible steps. Points are then awarded based on the proportion of the correct proof that you contributed.