I agree, it all depends on how well your externals go OP. Any estimate should be in a range for your case.
Tbh I would say ~90-99, yet I have to reiterate, it's your externals which are going to prove if you are going to even be close or in this estimated range.
In order to estimate an ATAR, you must provide your respective ranks for each subject eg. English Advanced- 5/25 etc and your school rank- which can be found on this website http://bettereducation.com.au/Results/Hsc.aspx?yr=2014