It is difficult to tell cut off marks because it depends on college, branch and home state quota also.

You should try to score more than 150 marks in JEE Main to have good chances in all the NITs.

JEE Advanced maximum marks are not defined. Maximum Marks vary every year like below

2017- 366

2016- 372

2015 - 504

2014 - 360

So, it is difficult to predict marks as exam pattern and marks change every year but you should score more than 40% marks to get good branch in Top IITs.

Good Luck!!