With just your score of 250, it would not be possible to predict your rank at this stage. Getting into an NIT will depend on the rank you get. This is dependent on the difficulty level of the exam and the number of students taking the test. So for now, sit  back and relax. Wait till the ranks come out. Having a look at the 2018 cut off ranks should give you an idea of which rank can get you into an NIT.