How much percentage do I need to acquire to get an admission in Lady Brabourne College or Bethune College in Kolkata for History honours?

It will depend on the cut-off marks as set up by the college. It differs from year to year. If you score above 85 your chances of getting selected in the first round will be easier. 

Later it will gradually drop down to 75-80. This will also vary among students of reserved and unreserved categories. Students belonging to reserved categories might require a lower score like 70-75, which can drop down to 60-65, depending on the course and the college’s set cut off.