You kind of get an idea of what sac scores/exam performance is required for each subject when you ask teachers/previous students/people that tell you their marks etc.
EG: I remember reading on VN of a few people that had A,A,A on their Maths Methods and someone with an A,B+,A, and they both got 35 for a study score. I think... ( assumption here) that most people will have a rough indication of what they're gonna get.
edit: umm and ranking wise.. also depends on the performance of those who may/may not have performed better than you on the SACs. (Basically, even if you're not ranked 1 and you beat them on the exam - generallly your score will be higher. ( assuming that the ranking gap is quite small)