If you're rank 1 in the cohort, it doesn't matter if you get 20% or 100%. From what I've heard VCAA doesn't care about the SAC scores, since some schools have easier SACs whilst other schools have harder SACs, so they aren't a good indicator. Instead they care about your SAC ranking and if you're rank 1 then you have the highest possible SAC grade you can at the school. That said since VCAA makes it so that rank 1 SAC marks turn into the rank 1 exam marks (the top exam mark in your cohort) and you just keep your exam marks. So, if you're rank 1 in a weak cohort and the highest exam score in your cohort is an A, then you get an A for your SAC score. Basically, the exact percentage if your SAC grade doesn't matter and if you want a 40+ you could get 20% throughout the year, but if you maintain rank 1 SAC scores and ace the exam, then 40+ is pretty plausible.
More about SAC rankings is in this link
https://atarnotes.com/forum/index.php?topic=160060.0