I don't judge; I found this a real hard subject, definitely had to spend far more time on it than any of my others to get a good mark!
Hmm, I personally don't think that you can tell study scores based on marks... If you keep up those same B-B+ scores, a 40 won't be possible. BUT, if you radically improve your marks, it'd probably work.
So, what's most important is focusing on improving your writing. What specifically are you doing wrong? Why did you lose marks in those SACs? Write out a list of the stuff you're missing out/not covering/doing wrong/could improve on; then, your study should focus on fixing up those problems. Hand in stuff to your teacher and ask for feedback; failing that, ask for feedback on your work here! Your constant focus should be - what exactly am I doing wrong, and how exactly can I fix that up? With that focus, rather than focus on marks, you'll really improve and that'll be your opportunity for a 40.