Hey there

First let me tell you that getting 75% or being in top 20 is the criterion of nits, iiits etc. It's not that you can't sit for jee mains or can't get admission into engineering if you don't fulfill the criteria. It is seen than State board has hard checking and so they have top 20 percentile system and thus if ypu are in top 20 percentile then you are eligible for jee mains.

Check the eligibility for JEE mains

https://www.google.com/url?sa=t&source=web&rct=j&url=https://engineering.careers360.com/exams/jee-main&ved=2ahUKEwj3uZrbrKXlAhU56nMBHRHvC0IQFjAAegQIAxAB&usg=AOvVaw1k3rMyr-Osb0qy4gsPfOaR

Good luck