1. In some cases it depends on your school, at mine, it's 12 units
2. Ignore how well or how badly a subject scales, as long as you're interested in it and you think that you'd do well in it, then choose it. There's nothing worse than forcing yourself to do a subject just for the scaling - ie. Chemistry - if you're doing poor in Maths or have a lack of interest in Science, you'd perform not as well as expected or end up failing the subject.
- Ext. 1 English - Scales highly
- Ext. 1 Maths - Scales highly
- Biology - Scales below Chem and Physics
- Chemistry - Scales marginally lower than Physics
- Economics - Highest scaling Humanities subject
- Ancient History - Scales below Economics, Modern and Legal but somewhat better in scaling compared to Business
Good luck!
I'm also submitting in my preferences in a day or so