There will be a standarized method used by the exam cell to prepare the rank list. The detailed analysis of the process of rank allotment is provided in the link below. Equal wiehtage is given to the marks alloted by the student in 12th standard and the marks obtained in keam which implies a 50-50 weightage. The marks of physics,chemistry and maths are to be taken into account for rank calculation and in case student has not studied chemistry then marks of computer science is to be considered and if in case none of the above mentioned subjects are studied by the student then in that case the marks of biotechnology is to be considered.

In my opinion , from records of previous years a mark of around 80-120 (to be on the safer side) can land you with the required rank of 30-40 k.

This link provided below will help you analyze the previous year rank lists so that be well aware from beforehand : 

https://engineering.careers360.com/articles/keam-rank-list

For detailed analysis please refer to the link as stated below : 

https://engineering.careers360.com/percentile-predictor-result/1136?s=64537830651554228785

this link will provide you with a rank preictor tool of keam developed by data scientists of careers360

ALL THE BEST.