Don't take this the wrong way, but I actually think that asking this question is really damaging to your ability to do well overall in Biology. Don't worry specifically about getting a 50 - just concern yourself with doing as best as you can.
A friend of mine was in the exact same position as you last year, and when he got a result he wasn't entirely happy with in a SAC he fretted about not getting a 50, and this really stressed him out for a long time (he had really high expectations). He ended up getting a 42 (which he was happy about by then), but in all honesty he could probably have done better if he hadn't been panicking about getting that 50.
Just focus on working extra hard for the next SAC, and keep motivated for the rest of the year. Biology's an epic subject, so make the most of it. As always, send a PM my way if ever you need help with something biol-related
