So if I got 80-90 through all my externals, there’s a chance of scoring between a 75-80 atar?
Well yes there's always a chance... Either way it doesn't matter, there's nothing you can do except to just try your best. Too many people on this forum asking if they'll get a certain ATAR, when in reality nobody knows. And it's not like their opinion is going to change anything in terms of how you study or what mark you get, so just give it all you got and see what happens when the results come.
All that effort people spend trawling through these forums asking for an estimate or if they'll get a certain mark is nothing short of a monumental waste of time; time that could've been better spent studying or doing something productive that helps them get to where they want.