More than worrying about marks, I suggest that you focus on the preparation for the exam. Check out this link for a few tips: 
https://www.careers360.com/question-im-in-12th-i-want-to-score-in-neet-but-i-havent-proper-knowledge-and-books-too-what-should-i-dohow-can-i-get-neet-related-pre-yr-paper?icn=question-title-details&ici=topic-detail.