My 10th, 12th,Bcom percentages are 85%, 78%, 67%. The least percentile I should get to get any of the 20 IIM calls.I belong to SC Category

Looking at your previous academic record, I believe that you can score a percentile of 90+ with good prep and even still higher (95+) if you take the right study plan. This, accompanied with the category cutoff, should easily land you a shortlist from the Top IIMs.

You should be assured that the percentile you require is lesser than that for General or OBC category. For IIM Ahmedabad, the percentile needed for an SC Candidate for a call is around 75 to 80. To be on the safe side, percentile above 80 can make you sure that will get calls from any of the 20 IIMs. From that point, it is mostly your skills,  how you perform in WAT,  GD and PI which will help you to get into any one of 20 IIMs.

Academic percentage of 10thand 12th do not play any role in CAT selection procedure. Graduation marks should be above 45% for an SC Candidate.

For IIM-A, The candidate must hold a bachelor's degree, with at least 50% marks or equivalent CGPA. In case of the candidates belonging to Scheduled Caste (SC)/ Scheduled Tribe (ST) and Differently Abled (DA) category, this is relaxed to 45%. The candidate may possess an equivalent qualification recognized by the Ministry of HRD, Government of India.

A min of 75% ile overall and a minimum of 60%ile in each section is the cutoff. But still to secure a seat in Top IIM's like BLACKI you'll need more. As the percetile decreases your IIM seat lowers down from Ahemdabad to Sambhalpur. At this stage your target should be to get maximum possible coz people in the same category will have scores above 90 also, so your competition increases.

Iim call k  liye pehle cat Mein Accha marks Lao,, thn call k baren Mein sochna