IISc decides a different cut off score each year depending on the difficulty of the paper, above which you are granted admission. This minimum score can range anywhere between 70-80% of the total (i.e 250-290), which makes it highly competitive. Such a score, depending upon the relative performance of students and difficulty of exam, roughly translates to a rank under 1500. Sometimes, a lower cutoff is also set, and students between this lower cut off and the former cut off have to appear for an interview. Hope this helps!