How is the aggregate(50%) of graduation calculated? Is it according to degree consideration (5th-8th sem) or total marks(1st - 8thsem)?

The aggregate of graduation shall be in accordance with your university consideration (5th-8th sem). i.e. 5th-8th sem. To confirm that, I'll suggest you to call the admission cell. You can get the number at www.balajisociety.org . All the best