Since the ATAR system is replacing OP in 2020, there are no scaling reports for QCE at the moment. However, since the subjects are similar to VCE, you can get a rough idea. Please keep in mind that it changes every year due to difficulty, so don't take these as solid numbers. Do pick subjects that you will enjoy instead of what scales best
According to the 2018 VCE scaling report:
Biology generally scales up by 1: median scaling.
English (mainstream) scales down by 2: low scaling.
Methods scales up by 5: high scaling.
PE scales down by 3: low scaling.
Accounting scales by either nothing or 1: medium scaling.
Digital solutions (we don't have this subject, but it is most likely equivalent to software development) scales down by 3: low scaling.
I wouldn't say these are super reliable, and I am not sure about digital solutions. Keep in mind that the higher your score, the less they scale. For instance, a study score of 30 in PE scales down to 27, however a study score of 45 scales down to 44. All those estimations are base on if you get a 30 study score (the average).