Hi in 70 to 80 marks you can expect around 84 to 90 percentile by seeing previous year trend but it depends on many factors like  paper difficulty of your slot and number of students appeared

You can check here

https://engineering.careers360.com/articles/jee-main-marks-vs-percentile/amp

For obc the cut off previous year was around 68 to 74 percentile this year may be changed depending upon number of students appeared