How to relate NTA score in main to the marks obtained.

Hi, the NTA percentile score is given after normalisation of your score that is after considering various factors such as the difficulty level of your paper. So you cannot relate your marks to NTA score, instead a easy way to understand or relate to your percentile will be that your percentile is the percent of students who have scored less than you after normalising your score. For example - let you have scored 80 percentile then 80% of the students who appeared have scored less than you after normalization.
If you want to know more about the normalisation procedure or how you can calculate your expected rank check out the following link-

https://engineering.careers360.com/articles/jee-main-normalisation-process-how-scores-are-calculated

All the best!