It's better if u have more than 80% because weightage of 12th std will be that main criteria in JEE Mains. In case if u get less marks in mains the 12th std marks will be added

Hello Y.D. Dash.

The following eligibility will be considered during admission, not during appearing in exam or filling the application form.

• For admissions to NITs/ IIITs/ GFTIs, candidates should have secured at least 75% marks in class 12th exam or be in the top 20 percentile in the class 12th exam conducted by the respective board. For SC/ ST candidates, the eligibility criteria is 65% marks in class XII.

Source:  https://engineering.careers360.com/articles/jee-main-eligibility-criteria

Hope it helps you and all the best! 

• Candidates have to score minimum75% aggregate marks(65% in case of SC/ST/PwD category) in 10+2 or equivalent examination.
  
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• Candidates must be within the category-wise top 20 percentile of qualified candidates in their particular 10+2 or equivalent examination.
  
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• Candidates, those appeared in 10+2 (or equivalent) examination in 2017 but reappeared in 2018, the best of two performances will be considered.
  

• All the best for your exam!!