Spectroscopy's spot on. All it takes is rank 1 and close to full marks on the exam to get a 50 study score. Doesn't really matter what the SAC marks are because they are subject to moderation by VCAA.
Basically, the SAC moderation system will try to adjust your SAC mark based on the other SAC marks of those who performed similarly to you on the exam/s. This is to minimise the effects of varying SAC difficulty across the state. So if you had 40% SACs, rank 1 and ace both exams, and the majority of students who aced the exams had 100% SAC marks, VCAA will scale your SAC mark up to around there also (and you'd get a 50). As another example, when I did Methods I scored around a 95% average on my exams, and VCAA found that most people who got 95% on the exams I did had 100/100 SAC marks, so my marks were scaled up to that. That's (briefly) how I understand the scaling system.
So raw SAC mark doesn't matter to much. If you're really gunning for a 50, ranking is the real important thing.