If you are personally not happy with your results, and you think you can improve, DON'T STOP. Don't settle with these scores if you know deep down if you tried and those results could get better.
High 60s is certainly a very good mark. The A+ for bio is actually pretty low compared to other VCAA exams. And you can afford to lose quite a few marks and still get your hands on a 50. That score is good enough to get you that 40+. But keep going! You have pin pointed your weakness (dating/ half life), attempt to go through it, if you are still having trouble, then consult a teacher or a friend etc.
Do not settle unless it's your best. As you said, you have put so much effort through out the year, you need to work even harder now because the end is so close. I hope all this hard work pays off. If you have your eye set on just achieving your absolute best, forget if it's a 35+ or a 40+, then I reckon you will absolutely astonish yourself come results day.
Good luck.