I am not really too familiar with the prediction stuff, but I have heard (and may be incorrect) that to get a 40+ score, you'd be looking at A+ A+ A+. Looking at the grade distributions, a low A+ on the exam last year was an ~80%. If that correlated to a 40+ study score, I don't know.
That advice is fairly useless I think. I think the better advice I can offer is: just try to beast the exam. Why not? Doing the best you can will give you the best score you can get.
It's kind of hard to tell if whoever marking these trial exams will mark the way VCAA will. Hell, some of them are obviously written in a style in a clear contrast with VCAA's style. Looking at assessor reports and the statistics on how people fare on IT exams, trying to perfect your exam technique to what VCAA wants to see seems to be crucial.
If you're getting high 80s to low 90s, you obviously know the content well. I don't know if that assumption is correct, but perhaps mark harsher and try to focus on perfecting the wording of your questions in order not to lose unnecessary marks etc. (of course continue to address all your other weak areas).
There's still plenty of time to before the exam to continue to improve.