how much average or estimate marks do i need to get into iit mandi (cs branch)
It will totally depend upon the difficulty level of exam because admission will base on your kee advance rank and fluctuate as per the difficulty level and students preparation. According to last year's results you should secure rank under 2000-2500 for cse in IIT MANDI in general category.
But you should do you best and try to secure a good rank .
Generally for an average difficulty level of exam I would suggest you to score minimum 60% marks in advance.