IF you are a BC resident, aim for a 76% to be safe. You will not get an interview if your average is in the low 70's. I know people who got rejected with no interview with a 73%.
If you are applying with a 70% again, don't even bother and save your money. If you are a current UBC student, you cannot retake the pre-req courses at UBC. You will need to do it again at a different institution. UBC SCIENCE only lets you redo courses if you fail (<50%).
Its not all doom though, there are people who get in with a 67% first year average, but these people have significantly higher last 3