To get a call from IIM A,B,C you need to get 98+ percentile or i can say 99+ percentile. Sometimes in blue moon a 98+ percentile gets a call.

Now coming to your Previous weightage then, it is totally fine and let me tell you for Initial calling these IIM's have different parameters of evaluation. Some gives only 10% weightage to Class 10th and 12th while IIM A give 20% weightage. In Final selection also IIM A give 8% weightage to previous records.