Your raw SAC score is only used to generate an internal ranking which is then converted to a numerical grade (and corresponding letter grade) by VCAA, who take into account cohort performance on the exam. Even if you know how previous years SACs were scaled at your school, SAC moderation does change from year to year even in the same school. If you did bio last year, you're going to find U4 chem a lot easier than U3 since a lot of the biochem stuff overlaps with bio U3. To do well in the exam, I'd strongly recommend dynamicscience which is a website that groups all past VCAA chem exam questions by topic, it probably saved my study score.
For reference, I got a raw 40 in chem with the following in 2018:
U3 SAC: raw 85% (scaled to 88%), rank 60 in a strong cohort of 110
U4 SAC: raw 90% (scaled to 96%), rank 5-10 in a strong cohort of 110
Exam: 195/240 (A+ cutoff in 2018 was 194/240, so barely scraped it. Usually a low A+ translates to a ~39 study score)