Mr. Nagendra,

The eligibility criteria for taking writing CET Exams of a state is that the candidate appearing for the exam should be under one of the following categories:-

1) He or She should have studied for a minimum of 7 years (1st std. to 12th) in that state as of 1st of June as the year of the exam and should have studied and passed from a recognized institute from that state.

2) Candidates who passed their 10th or 12th exam from the state are eligible too.

3) If the candidate has a nativity certificate of the state or if the candidate is a domicile of that state, then he or she is eligible for the exam.

Refer this link for further doubts, it will help you out totally - https://engineering.careers360.com/articles/kcet-eligibility-criteria

To calculate the Karnataka CET Rank - 

You will have to consider 50% of your Physics, Chemistry, and Math marks in 12th and 50% of your KCET Marks as well.

E.g.

If you score 60% in 12th PCM, then 50% of that would be 60/2=30%.

And if your KCET PCM marks are 65%, then 50% of that would be  65/2=32.5%

So, your rank will be calculated based on your total average, 30%+32.5%=62.5%

And this average should give you a rank inside 15,000.