Yes you may get a seat in Sastra University. Because 70% of seats admission to Sastra University depends on both the criterias ie the boards marks (75% dependency) and also the JEE mains score (25% dependency). So your percentage is 97.7 right in 12th boards. So I'll show you how your final score by sastra university will be calculated.

Your score * 100 / topper score

97.7 * 100 / 98.7 = 98.99  (assuming toppers score is 98.7)

Now 98.99 * 0.75 (boards weightage) + 85 * 0.25 (JEE Mains weightage) = 95.5

Now this is your final score and at this score there are very high chances that you will get into Sastra University pretty conveniently.

Best wishes. Thank-you.