You can still do really well.
If you think about it, all the SAC marks for the year are worth a combined 34% of your overall mark. So if you do a couple of SACs in each module you'll be doing, say, 8 for the year. So that one SAC that you just had was worth 12.5% of your mark for SACs. Which is 12.5% of 34% for the year - 4.25% of the overall study score. Not very much, in other words. If you scored an 80 and only lost 20% of the marks, you only lost 0.85% of your year's mark. Hardly anything. So don't stress. Just be careful to check the number of pages in future, and if you kill it in the other SACs and exams you'll still be in the top of the state!
Plus, if you end up getting 75-80% that's still a fantastic grade and you should be proud of yourself! Keep up the good work!!