Let me tell you that my friend with 80+% in tenth, 58% in twelfth, and average percentage in graduation (Engineering) got into IIM C with 99+ percentile with Exceptionally Good Interview.

But, I would say if you can manage and get a 99+ percentile, you will definitely get into one of the IIMs in the country.

Also, to get into an IIM, you need to perform in not only CAT, but also the GD/WAT/GE/PI as part of the selection process once you receive a call from them.

I am confident that if you start practising for the test as soon as possible, you can definitely get through. But, at the same time explore other tests like SNAP, NMAT, CET, IIFT, MAT, etc. This will open up a lot of options for you to explore if in case you don't make it to the IIMs.