Hello,

Top 20 percentile is only for IIT-Jee admission. It is not for any other state board. When they consider mark they will frame your mark into their standard and then they will calculate. Lets say if your one paper total mark is 80 then that will be framed into 100 standard then they will be added so that it won't be unfair to other students too. Every state board is not same. The have different patterns.

For considering top 20 percentile, they will consider your 3 major papers which are maths,physics and chemistry then other two can be any subject in which you scored highest.

For more details you can visit:- https://engineering.careers360.com/articles/top-20-percentile-of-class-12-boards-for-nits-iiits-and-cftis

Hope it helps!