how i know i am in the top 20 percentile
You can check the top 20 percentile of previous year for NITs & IITs below -
https://josaa.nic.in/webinfocms/Handler/FileHandler.ashx?i=File&ii=86&iii=Y
https://josaa.nic.in/webinfocms/Handler/FileHandler.ashx?i=File&ii=87&iii=Y
Good Luck!!!