No you shouldn't since this is the progression of the HSC:
So you sit your HSC exams and then come sometime December the HSC results will be announced via BoS (the other one), usually they will come in the form of two categories of equal weighting:
50% of your mark will be internally based. Thus this will be the accumulation of all those assignments, essays, tests and whatnot you did in year 12. Trials play a large component since they are usually worth something like 40% but then again remember that you have the other 60% of internal marks to go - trials are not the end of the world. I've found that the mark on the HSC results is the same as the one the school gives you - there is no scaling*
The other 50% of your mark will be the HSC exam result. With this you only ever get to see your scaled mark, not your raw mark. Usually most HSC subjects scale up so this shouldn't be a matter. In real life most of the class of 08 who did 4-unit math at Sydney Girls thought that they had failed the exam - it was that hard. But upon receiving their HSC exam mark found that they still achieved a decent result.
And that is how they calculate your overall HSC mark, its just 50+50 = 100
As long as you do well in externals, you'll be fine.
Hope this helps =P
