Maths -100% , physics and chemistry -50% of your scores.. so if u get 100 in maths, 90 in physical, 80 in chemistry, your cut off will be -100+(90+80)/2 =185. And if you mean the cut off that is set by the college for you to be called for the aptitude test, its based on these competition. The college calls a reasonable no of students for the aptitude test. So if there are more no of people scoring very high marks (more than you)in the year you apply ,then the city of will be high and maybe you will not be called. Its dynamically decided every year but you can rely on previous years trends and hope for the best!

Good Luck