To be honest, your class 12th marks doesn't matter that 

much as they only need to make you eligible for a particular 

exam.

But, you'll have to work pretty hard to offset that 12th score.

To be good enough for you will most importantly need 99.5+ 

percentile in CAT and good enough marks in your graduation 

too.

Though you have to keep in mind that there are some top 

IIMs which are strict for their 10th, 12th criteria, do not 

keep many hopes from them.

Good luck!