@matt flynThere is no mark or percentage you want to aim for. No such thing.
You want to secure the best rank possible. For a 60ATAR, at a school ranked around 200, you'd want to be in the upper average end of your cohort.
Plenty of study tips on this site, and threads that explain scaling amazingly.
But in short to answer your question, If your cohort has a higher % of Distinguished Achievers, your internal mark should end up being higher.
I'll try and say this simply:
Your INTERNAL mark for your atar is the HSC mark (external exam) that your rank yields. So say you rank 7th in your cohort for english, you get the 7th highest HSC mark for your internal mark (and ofc your HSC mark is your own). So naturally, if your cohort is smarter, and they achieve higher marks on the HSC exam, you'll do better.
But really don't worry. Doesn't matter what school you go to. If you gun it and rank 1st (just say), and smash the HSC, you'll be sweet as.
Also, if you're wanting a 60ATAR, don't aim for it, aim higher. People rarely achieve the mark they want in the HSC.