Hello student with 40 marks in Jee main your percentile will be around 70 percentile.

You can calculate and get your rank by using this formula

100 - (your total percentile) 869010/100

In your case your rank must be around 2,60,703, this might vary as per the total number of applicants.

Cut off vary every year on various factors like the level of question in the entrance examination and total number of applicants in that examination.

For more updates refer to the link mentioned below - https://www.google.com/amp/s/engineering.careers360.com/articles/jee-main-marks-vs-percentile/amp

Feel free to comment if you have any doubt

Good luck