Hello Rajeshwar !! so.. you just need to have 75% or to be in a top 20 percentile in board examination with good score in jee mains is required to get admission in nit but If you are sc/st cateogory you just need to get 65% in board examination.See.. now a days jee mains rank is given importance than board exmination percentage 60% is for jee mains exam and 40% is for board examination. so you dont worry and be prepare well for the  jee main.Getting 75% and above it will become easy if you prepare for jee mains. Hope i might have helped you .All the best!! prepare well!!