Hey Abhishek.

The top 20 percentile for Bihar board is not out yet. In the previous year, 2018 the top 20 percentile in Bihar board for each category was as given below. You can expect a similar range of cut off to presume as of now. For 2019 it will be released soon

GEN 293

OBC NCL- 287

SC-279

ST- 284

PWD- 279

Best of luck!