Cool. Let me answer step by step.
You don't know mark differences - No worries, we won't worry about it then.
You do past HSC questions for physics and chemistry - Brilliant.
Do you ensure you understand what you are answering and not writing down content you just learnt mindlessly? Do you also check the marking criteria and identify what you covered, and what mark (and hence what band you get)? If yes, excellent. Otherwise, it may be time to send responses to your teacher (assuming he/she is good) and asking for feedback. Or you can post it in a thread (the question thread, for now). Whilst I'm not really near a 'qualified marker' anymore I'm sure Jake will be able to help, and I can also provide additional feedback.
You do past papers for mathematics? Perfect. Nothing much needs to be asked here apart from how you go with the last questions on the paper.
You prepare the essays for economics? This is great, provided you know how to adapt your response. However many essays you actually write, but at the end of the day you must have the capability to adapt them to the question given. (Basically the same as English in this regard.) Also make sure that the multiple choice is never neglected. One of my friends who got a 91 in economics has told me that the multiple choice tends to be the hardest part in the course.
Unfortunately at this point in time we do not have members that focus on economics here, so we might not be able to provide quality feedback just yet... 
In regards to ATAR calculators - Your marks undergo two processes called moderation and alignment. Moderation is used to cut off the fact that some schools set harder assessment tasks than others, and standardises it with respect to the difficulty of the final exam. Alignment just caters for the fact that some exams are harder than others in different years. Your mark that you should put into the ATAR calculator is your
aligned mark. I recommend you predict your raw mark in the exam, use the
raw marks database, and proceed. (Of course, the ATAR calculator is only truly accurate the day you get your marks!
Finally, ok so the thing you must realise is that you have, to some valid extent, satisfied the criteria for assumed knowledge. It's not a prerequisite; it's assumed knowledge. This means that you have learnt the content; just not been properly examined on it. (Bridging courses may be worth considering but they do cost after all.) I recommend that if you feel you're going to
waste too much time on MX1, get rid of it. Otherwise, yes feel free to discuss it with the staff at your school