Hey I'm going to sound like a total hypocrite for writing this up after the rant post on my journal but hear me out:
The exams are worth a lot more than your SACs. I don't know where the ATAR calculator will land you with predicted study scores (and I'm not experienced enough to comment on that or answer your real question on whether or not you will get a 90 for sure) but I'm aware of so many people that were doing really well in their SACs and went down because of their exam score, even people whose SAC averages were much, much higher than mine but they stuffed the Exam and ended up with a lower study score than me for Bio.
I know it sucks to get a low sac score but please don't call it "unacceptable". I'm sure a lot of work went into those scores and it's simply unfair to yourself to deem your work as unworthy like that. It's just how well you were able to answer a bunch of questions in a very restrictive environment, there's no "acceptable" or "unacceptable" standards, please don't treat your scores like a behaviour that you can control. Your scores aren't you. If you know you've worked, give yourself credit.
There's certainly all of U4 left, plus the exams. The spesh exams are worth 66% of your grade, and assuming you haven't sat your u4 sacs yet this means that you've got 88% of your score still unassessed - isn't that a little too early to give up?
And even in the case that your SACs don't go as planned, don't be harsh on yourself. There's certainly pathways that can get you to your required course, regardless of your ATAR. It might take a bit longer but if you've tried your best and the score you get doesn't seem to reflect that, it's okay. VCE is a very specific system and it doesn't cater to reflect everyone's ability. That's a flaw in standardised systems and as long as you give it your best shot it's okay.
So please, don't give up just yet and be kind to yourself. There's heaps of time for improvement.
tl;dr: you could get a 90, or you couldn't. It's too early to give up.
Good luck!
Nelope.