It is not based on a percentage. It is dependent on the level of difficulty of the questions you are getting right/wrong. Get more questions right and the difficulty of the questions are supposed to get harder. It is checking to see if you are definitely above the passing standard or definitely below the passing standard.
You can go to this website and look on page 15 for further information on scoring: https://www.ncsbn.org/2009_NCLEX_Candidate_Bulletin.pdf
As said by LisserRN, NCLEX is not based on percentage but the test is base on level of difficulty question. My own theory of surviving NCLEX is to have 68-75% on practice test. I came across a replica CD like NCLEX format which i think prepared me before taking my NCLEX.