There is no specific percentage or grade requirement for candidates in their 12th standard to be eligible for the NDA (National Defence Academy) exam. The eligibility criteria for the NDA exam are based on age, nationality, and educational qualifications, which are as follows:

1. Age Limit:
- The candidate must be between 16.5 and 19.5 years of age.
- The date of birth should fall between January 2, 2003, to January 1, 2006.

2. Nationality:
- The candidate must be a citizen of India.
- A subject of Bhutan or Nepal can also apply, provided they have an intention to permanently settle in India.

3. Educational Qualifications:
- For Army Wing of NDA: The candidate must have passed the 12th standard or equivalent examination from a recognized board or university.
- For Air Force and Naval Wings of NDA and 10+2 Cadet Entry Scheme at the Indian Naval Academy: The candidate must have passed the 12th standard or equivalent examination with Physics and Mathematics as compulsory subjects.

Visually appealing representation:

Eligibility Criteria for NDA Exam:

1. Age Limit:
- The candidate must be between 16.5 and 19.5 years of age.
- The date of birth should fall between January 2, 2003, to January 1, 2006.

2. Nationality:
- The candidate must be a citizen of India.
- A subject of Bhutan or Nepal can also apply, provided they have an intention to permanently settle in India.

3. Educational Qualifications:
- For Army Wing of NDA: The candidate must have passed the 12th standard or equivalent examination from a recognized board or university.
- For Air Force and Naval Wings of NDA and 10+2 Cadet Entry Scheme at the Indian Naval Academy: The candidate must have passed the 12th standard or equivalent examination with Physics and Mathematics as compulsory subjects.

It is important to note that although there is no specific percentage or grade requirement mentioned, candidates are advised to score well in their 12th standard examinations as the competition for the NDA exam is high. A good academic record can enhance the overall profile of the candidate and increase their chances of selection. Additionally, candidates must also meet the physical and medical standards set by the NDA to be eligible for selection.

In conclusion, while there is no specific percentage or grade requirement for the NDA exam, candidates must fulfill the age, nationality, and educational qualifications criteria to be eligible. A strong academic record can be beneficial in the selection process.