what is the minimum mark and rank required to get a good NIT?

Hii

Prediction in terms of marks is quiet difficult,  as marks distribution changes with difficulty of paper every year. I can help you in terms of rank.

For top NITs-

You can get CSE or ECE till 5k rank and core branches till 12k rank. You may get lowest branch upto 17k at last.

In average NITs-

Getting CSE till 10k is very common and core branches till 22k rank. Any how you may get lower branches there till 30k rank.

In lower or northeastern NITs-

You may get admission there for CSE till 15-20k, and all other branches till 30-35k rank.

All the above rank were estimation and may vary somewhat.  The all rank were represented in terms of general category rank.

Hope it helps