Hello Prabha!

Try to keep your score around 200. Look the Marks vs Rank trend varies highly year by year. If you were have been the candidate of last year then 170 is enough to get IIT. But if you were a candidate of 2017 then 200 is needed at least. 

Focus on doing only those questions which you know. Don't look at the needed score for IIT as of now. Stay calm, focus on your exam and just do those questions which you are sure about.

All the best!