Hi!
Welcome to the forums
Since the scores are reported out of 100 I can understand the temptation to think of them as a percentage, but getting 90% on all of your assessments does not guarantee you a score of 90 in that subject. Internal scores can be added to find your overall final internal score but external assessments are a bit different.
Also going to jump up to say that yeah the results you get this year do influence your options after highschool and therefore your life but it's not like this is
the moment which sets you on one path and then you're stuck on there. I aimed for a 98+ ATAR and got it, but tbh aside from working in tutoring etc. my life would not have been very different had a gotten a bit under 90 instead. I like my course, it's unique, & being an advanced science course it doesn't have the lowest ATAR ever but I still could've gone on to have a similar career path without it.
Aim for a high ATAR imo because it's nice to have the freedom to have more immediate options and not need to take a different path but it's not going to ruin your life if you get a low one.
As for the ATAR prediction, I haven't pulled the data off the report and I don't know of others who have so the best estimate I could give you would be using the calc but w/ the report's scaling - which it sounds like you've already done.
There's going to be some variation depending depending on cohort performance so keep in mind that no estimates for the 2021 cohort are able to be guarantees - even once the 2020 data is (not just scaling but used it'll refine it but no calc will never be perfect. That being said, last year the median ATAR was 78ish so 90 might not be as far away as it feels