Hi aspirant ,

you can have chances in getting under 20 percentile . till know the percentile is not been released  but i can tell you about previous year marks need for 20 percentile like in 2017 401 marks and  2018 384 marks . NTA will release this information on NTA portal after 15 july after cbse class 12th result . But i you not come in top 20 percentile then 75 % in board exam is necessary to get eligible for nits and iits . But you have a high chances  that you can come in top 20 percentile .