Yes, any candidate having more than 55% in General/ OBC category is eligible to appear for JAM exam as mentioned in the eligibility criterion follow this link :

http://jam.iitkgp.ac.in/?id=p&n=306

However, Enrollment for JAM 2019 is closed as Mentioned in the official Website.

Hope You Got Your Answer.

Look it can't be predicted as cutoff depends on a lots of thing above that subject wise prediction is next to impossible as overall marks make a difference which is a sum of three subjects you have to score in total 

Cutoff is different for all NIT through CCmN in jAM exam. Cutoff is different for all streams as well. 

Detailed cutoff reports for all three rounds shared below for reference  

https://university.careers360.com/articles/ccmn-cutoff

Hello Aryan

It would be difficult to provide you the exact marks required as the cutoff vary every year depending on various factors. However, previous year closing rank of IIT Roorkee for Physics was 221 for General category, 378 for OBC NCL category.
You can go through the link below

Hello,

Your aim is clear. The above mentioned academic and careers options are best if you pursue them with passion.

Good luck. Best wishes.

Here are few preparation tips you can follow for the same.

Begin Well with knowing the IIT JAM Syllabus & Paper Pattern.
First of all, you need to know the IIT JAM Syllabus & Pattern. As you all know the syllabus is huge so start preparing few months before the

You shpuld understand exam pattern and syllabus well and make timetable and be prepared well before 3month of exam.

Last 3months just appear for mocks and analysis of mistakes is must. Revise concepts and make notes for revision. Focus on tough concepts  

Maximise time towardsgroup solving doubts and revision and

This depends on the college you are applying for, some college give admission on the basis of your class 12th marks and some on the basis of entrance exam they organise so if you are applying for the colleges which gives admission on the basis of class 12th marks then