For getting admission into IITB you must have score above 850.Even that does not guarantee your admission. But if you don’t get admission with 850+ in IIT Bombay it means that most of the top rankers have chosen IITB. In this case IISc seats remain vacant and you can join IISC(equal or better than IITB for M.Tech CSE).
This year the last rank for IITB admission was 86. My rank was 67 and I had a score of 878. So 86 rank means a score of around 850.