In Engineering, your recent achievements and ranks are more valuable than your past academic performances. Unless you have a very low CGPA, your class 10th and 12th marks won’t affect your placements. However, your class 12th marks do determine your JEE rank and the branch you get which in turn affects your placements. But it’s important to note that this is a very short term effect and 10 years down the line, neither your branch nor your class 12 marks will affect you. For interviews, prepare a solid credible reason to justify your low marks, and don’t hide it.
Now, coming to your question, how much is needed in class 12 to get into IIT Kanpur, well there are no fixed marks that will get you into IIT Kanpur. But you can rest assured as the board exams and JEE papers are much similar and you don’t have to invest extra time or efforts preparing for them separately.