yeaaaah i so aint getting a 40 
Surely my SAC's will help? 50 percent of the final grade?
what do you mean? 
Not 100% sure but I'll give it a go.
I'm going to use an extreme example
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Lets say you got 100% on all sacs, and get 1% on your exam.
Your not going to get that 50% worth of sac marks to your SS. That 50% will be scaled down.
Essentially what I'm saying is that even on the VCAA website it says Sacs are worth 50% of your mark, in reality Sac marks are no guarantee.
But dude don't worry, I checked VCEIT.com and I've made countless small stupid mistakes even though the exam was relatively easy. I'm pretty much in the same boat as you, looking at high 30's low 40's even though I'm rank 1. I believe I got 80% for the exam. What's done is done.