The thing is, your school is ranked 170. Whether it fluctuates significantly OR remains around that 170 area cannot be definitive OR a good indication of how well the cohort of your school is. TO secure a 99+ ATAR, you'd want to be ranked 1st or somewhere close like 2nd or 3rd in all your subjects so that other students don't hinder your chances of attaining your desired ATAR.
OP, imo, forget about trying to workout what rank you need, because it can result in complacency. For instance "OH 2u maths cohort is pretty good, all i need is to come 10th because everyone else will get band 6 and they won't effect me" YOU DONT know this for sure, and there is a chance someone can fuck up.
SO what I'm basically saying is, try your best, put 100% in and try to come first!