Hello Aspirant, First of all I would like to say how top 20 percentile of any board is calculated, Top 20 percentile of any board is calculated by marks scored in five subjects which are

Physics

Chemistry

Mathematics

Language subject

Any subject other than physics, chemistry, Maths, language paper

If any body has taken 2 language paper then higher marks scored subject will be taken for calculating top 20 percentile.

Top 20 percentile of any board is announced for a year after the board results of that year is announced.

Now, Top 20 percentile of Bihar board for last few years are(marks out of 500)

2016

General Category - 315
OBC Category - 308
SC - 297
ST - 300
PWD - 297

2017

General Category - 313
OBC Category - 307
SC Candidate - 300
ST Category - 303
PWD category - 300

2018

General Category - 293
OBC Category - 287
SC Category - 279
ST Category - 284
PWD - 279

2019

General Category - 346
OBC Category - 347
SC Candidate - 332
ST Category - 339
PWD - 332