ok, heres how it goes.
your UAI is based on your overall mark.
your overall mark is 50% school based assessment, 50% hsc
so say your school based assesment is 75% and your hsc mark is 60%, your overall mark would be 67.5
they then take all your overall marks for each subject and scale them and do other funky things that noone (not even them, i dont think) understand to create your UAI