Calculating the percentile is easy by following this formulae:

Percentile = (Number of values of x/Total number of values)*100

Now for example, in an exam there are 5 students have scores 50,60,70,80,90. Now if you want to calculate the percentile against score say 82. Then,

Number of people score below 82 is 4. Total number of values is 5.

Thus the percentile against 82 score is 4/5*100= 80 percentile.

Now if you want to know specifically about NTA percentile score calculations in JEE MAINS visit the link provided below:

https://engineering.careers360.com/articles/jee-main-marks-vs-percentile

Now to make this process more statistically accurate as well as easier for you, click on the link below:

I hope my answer helps. All the very best for your future endeavors!