I don't think, you can calculate marks from your percentile. Percentile is the percentage of people (who appeared for that exam) whose scores are less or equal to your score. If your percentile is 81 that means 19% students are ahead of you. And this year 3.93 lakh students appeared for MHT-CET, so 19% of 393000 gives you your rank. But, you cannot deduce exact marks from this rank or percentile.