You should not certainly put 75% in your JEE Main application form if your actual percentage is 74.8%. The reason is,

Honesty and Transparency: Filling up the form with the wrong information is a type of discrepancy, which can make you liable for disqualification at the later stages of the process.

Verification Process: While getting counseling or document verification done, 10th class mark sheet shall be checked. Mistakes due to the variation of the mark sheet with an application form will result

Now about NIT counselling: -

Cut-off percentages depends upon the final score for NITs; when counselling will be done they do take eligibility based upon percentage scores achieved in your board final result of class XII.

Round-Wise Counseling: NIT counseling happens in multiple rounds usually. You can improve your JEE Main score and even upload documents in the later rounds if you are shortlisted based on your increased 12th board percentage.

Here is what you can do:

Use the Actual Percentage: Fill the application form with your actual 10th board percentage of 74.8%.

Study Hard and Score well in the improvement exam so that at least 75% will ensure NIT eligibility

Monitoring Counseling Rounds and Cut-off release dates

Prepare for Counseling if you are able to secure at least 75% marks in your improvement exam.  You may be asked to submit documents and subsequently undergo counseling rounds.

The following links will be helpful in this regard:

Official website of JEE Main

NIT Admissions: https://engineering.careers360.com/colleges/list-of-engineering-colleges-in-india

Never forget to be honest and accurate during the application process. Improve your board percentage and attend the counseling round that you qualify for.