Hello, you can use the below formula to calculate the rank but it is very difficult to get your exact rank

100 - your total percentile score is the percentage  of people ahead of you. 869010 are the number of candidates appeared for Jan exam. So rank formula is (100 - your total score) X 869010 /100. If the number of candidates for April exam increased to 1200000 then your final rank shall be (100 - your total score) X 1200000/100

Though this is not the exact rank you will get but your rank will be close to it. it’s difficult to tell exact qualifying marks for jee advanced eligibility but if you are getting more than 85 percentile score then you have decent chances to qualify for jee advanced