78% won’t be enough to get you into IIM Shillong. You need at least 80% in your 10 and 12 grades. So we can say that for the final selection you must have 98 percentile. Other criteria to be considered are:
For engineers: Score of above 84 or 85 in 10th and 12th with min 8.0+ in undergraduate
For non-engineers: Score of above 80 in 10th and 12th with min 7.5 in undergraduate
This criterion is subject to change every year. Other factors that affect admissions are:
Your performance in the interview
Gender
Undergraduate stream
Both the IIMs will require above 85 to get admission. The eligibility criteria for IIM Kozhikode are a bit more strict than that of IIM Shillong. The minimum overall and sectional cutoff required for admission to IIM Kozhikode will remain 85 percentile and 75 percentile respectively for General category candidates.