Assuming for Chem, around the top 20% of students get A or above.
If you keep this top 20% through out your SACs and Unit 4 Exam, your scaled score will be at least 40.5. (this is calculated using the statistics for 2010 Chemistry in the 2010 Scaling Report where Mean=34.44 and S.D.=7.2)
This translates to a raw score of around 36 or more.
You learn how to calculate this in the last chapter of Methods (Scaled score=invNorm(0.8,0,1)*7.2+34.44)
*Got bored so did more calculations. An A in Physics translates to a raw score of 35 or more. So it is actually easier to do good in Chem than Physics (since physics is easier I guess). Also constant A+ in Chem translates to a raw score of 40+. This is A+ overall, so since you got an A, and assuming you are getting A+ in your SACs (matters less if you have a strong cohort), you will need at least a high A+/low A or low A+/high A combination for your two exams to get 40+.