beats it's good to est from year 11 but it's not completely reliable... Why? Because circumstances can dramatically change and the topics expand over time. I performed really well in year 11 and loved all my subjects, but burnt out right around trials. This procrastination and lack of drive continued to the HSC (I've also faced other challenges). Despite ranking first and having good internal marks, I'm estimating a 60-70 atar (my class average is bad... gone extremely down this year). My advice: time management, practice and study wisely. Even if your internal marks are bad, there are many stories of people getting 90s because they smashed the HSC.
And btw I est at the beginning of year 12 an atar of 92
good luck^^