it is based on what everyone from that school goes on to get in the HSC because every school has different exams. I'm pretty sure they take 3 anchor HSC marks form the cohort and base the internal marks on that with respect to the relative difference in marks. so your internal mark depends on...
the school doesn't send a single mark to nesa only your rank and the mark difference between you and your peers. the mark you get for you internal matters entirely upon how the marks your cohort gets in the HSC and your position within that cohort.
Id say to do some of the working out for calculations in your book since if you are going to have an open book test it is highly likely they're gonna test on calculations