Messiah,
The relevence of the details of my operation is that you implied what is being discussed here is somehow uniquely Qantas. It isn't.
I'm afraid you still don't get it. Are you saying that, having used the approved standard weights for pax, baggage etc and having an indication that those weights plus the fuel your gauges show puts you over your Max Takeoff, you attitude would be "Well, they're only estimates, so to hell with it!"? By how much before you decide it's too much? One Tonne? 5?
As to legalities, you are LEGALLY OBLIGED to operate within the terms of your ops manual. Does yours say "Near enough is OK" or "Use the approved loading system". If you are operating outside the letter of the book, you are on your own.
The only "Good faith" information you have is the approved loading system. Ignore that at your perlil.