Most are pretty accurate you just need to be careful when adding in your scaled marks. For example, let's say I get 90 raw in English which usually scales to about 93, but my internal rank isn't great 20/30 and the cohort is weak. Then I'm not going to put in a 93 into the calculator, rather I'll have to estimate that my rank (50% contribution) might be a hsc mark of 83 which means my overall HSC mark is 88. This will ultimately make a very big impact on the final result whether for the better or worse depending on your ranks. However, if you have the honour of being the only student or just 1st in every subject then you can just estimate your raw marks, and scale it using raw marks database website, and the atar projection will be pretty accurate.