There is no fixed percentage to enter a particular stream.
However, science stream requires a good score to get an entry.
If you are aiming at a good college, then a score of above 80 - 90 % is a must as the competition will be more.
If you are looking for any college to get into PCB, then a percentage score of above 60-70 % should be enough.
This is just a statistical data and can vary from place to place and college to college.