The minimum score to get a good IIT varies greatly every year. And moreover the definition of good IIT is very hard to say. But I believe all IITs are good.

It's always safe to score above 40% score in advance to get a good IIT and a good branch.

A rank close to 300 needs 167 marks and can assure that till 4000 ranks you get top IITs and best branches. So, always make sure to score close to 50% to be extremely sure.

Hope this will be helpful.

Best of Luck!