Firstly you had done good in class12th! And for jee mains rank depends on how other students had performed in the same . But for rank 20 your marks should be 250+

Hi aspirant 

thanks for asking your query as to get rank under 20 you have to be very hard working because such rank requires alot efforts and your percentile should be in 99 to 99.8 or minimum marks of 300 but direct prediction of marks and percentile is futile because every year paper pattern changes therefore to score such marks you need to focus in studies only.