Based on SASTRA University's admission process, your chances of getting a seat depend on how your JEE Main percentile and +2 marks are considered together. SASTRA University uses two streams for B.Tech admissions:

Stream 1: 70% of the seats are filled based on a combined score, giving 50% weightage to your JEE Main normalized score and 50% weightage to your normalized +2 marks.

Stream 2: The remaining 30% of the seats are filled solely based on your normalized +2 marks.

With a 76 percentile in JEE Main, you have a decent score. Your +2 marks of 971 are also quite good, assuming they translate to a high percentage.

To assess your chances for Stream 1, SASTRA will likely normalize your +2 marks across your state board and then combine it with your JEE Main percentile (after converting it to a score) with equal weightage. A good combined score increases your chances.

For Stream 2, your high +2 marks are a significant advantage as admission is solely based on this. If your 971 marks correspond to a very high percentage, you have a strong possibility of securing a seat through this stream, even without a very high JEE Main percentile.

Therefore, with a 76 percentile in JEE Main and high marks in +2, you have a good chance of getting a seat in SASTRA University. Your high +2 marks, in particular, will be beneficial for admission through the 30% quota based on board exam scores.

It's advisable to apply to SASTRA University and check their admission guidelines for 2025 for the exact weightage and normalization process they will follow. You should also look out for their rank lists after the application process to know your admission status.