sac marks are moderated in light of your whole schools exam performance, so very basically they go down if your school doesent do as well on the exam (indicating that sacs are easier than at other schools) and they scale up if your school does better on the exam (indicating that the sacs were more difficult than other schools) and after all this, you get a sac mark that is fair in comparison to the rest of the state because sacs are not standardised, so they need to use exams (and a tiny bit of the gat, i think) to judge how you really went all year.
its a pretty complicated system when you get into the technicalities, but basically your 50% from sacs will be moderated a tad due to your cohorts exam performance. you dont get 'dragged down' by a weak cohort, though, your own performance has a lot more effect.
the thing is, everyone in the state is subject to this moderation and as a result its really wuite hard to tell what study score a certain exam/sac score will get you. I would stress less though, because firstly the exam is done and you should be spending your holidays free from vce, and secondly because youre looking at a high 30's/low 40's for the subject (you may have even done better than you think) (eg with A+ sacs and a low-mid A exam i got 39 in legal last year, after the exam i was expecting a 33-35 at best)
the point is, nobody can accurately guarantee you a study score, you just have to wait it out and see what happens, it may even end up better than you thought