Hii aspirant..

Every school board in India have their own criteria for deciding top 20 percentile.

Thus if you want to check that whether you are in top 20 percentile or not I would suggest you to check it on the official website of jossa.

For reference let's take an example that 60% of Jharkhand Board is equivalent to 80% of Bihar board thus each school board state board CBSE ICSE and other boards provide there top 20 percentile to jossa and if you want to know then you can check it on the official website of jossa.

You can know more about it visiting the following link -

https://engineering.careers360.com/articles/top-20-percentile-cutoff-of-class-12-boards-for-iit-admission

Hope it helps. 

Best of luck.