It's really hard for us to predict what study scores you will achieve based on that information as your rank in your classes is far more important than your percentage scores. Are you averaging in a class where most people will do well on the exam? Are you below average in a cohort that will probably perform below the state average in the exam? How well do you perform on practice exams? etc. are much more relevant questions for study score calculations.
(I know this probably doesn't seem to make much sense initially, but VCAA doesn't really care about your raw SAC marks). There are a range of explanations and predictive techniques
here if you want more info on how the prediction process works
My rough guess is that you'll probably be aiming for a B+ in your exams to get a 70