Hii Diwakar ,

Coming to your question the top 20 percentile of up board giving you a rough idea and the last year stats is ,

If you are a general category the last year top 20 percentile was 338 in the 2018

It was 355 in the year 2017.

So basically the top 20 percentile depends on the difficulty of paper pass percentage of students and many more

For more information you can go through the link stated below.

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

Hope this helps you ,good luck