Unit 1/2 is all internal assessments, whereas unit 3/4 is all external assessments (the internal SAC's you do get scaled externally), which makes it practically impossible to predict your ATAR based off your unit 1/2 assessments.
The best you could do would be to find a year 12 student at your school who is getting / used to get exactly the same marks you got in the same subjects, then see what their ATAR is. Though I can't guarantee that such a person exists. And even then, this assumes that your scores and their scores are consistent from year 11 to year 12, which is a very big assumption.
apparently at the end of yr 11 some schools give their students a predicted atar
I've never known any school to do this... I know that some teachers will give you a predicted study score for a particular subject towards the end of unit 4 by looking at your SAC scores and your trial exam marks, but that's about as good as it gets.