Hi, Having no work experience is okay because it is not a mandatory requirement. if you have abive average past academic record then you can get expect a rating of 4-5 out of 10 (scaling criteria varies in all IIM's) 

you will be required to score above 99 percentile to get into old IIM's and for new ones keep a aim above 94-95 percentile in case of general candidate.

Your profile and your performance in the other rounds of the interview will be a second major impact after the CAT percentile. So make it impressive or else you lose the chance. You should be able to justify as in why your score is average if asked.

Good Luck.