Hey gaurav.
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
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