Another thing to understand is diminishing returns. Keep in mind that the aggregate runs on a linear scale, while the study scores operate on a normal distribution. The importance of this is that the time spent getting a certain study score increases exponentially as your aim goes higher (since the study scores operate on that sort of scale). For example, getting a 30 isn't particularly difficult, while getting a 40 might take double the time spent, but then furthering this to a 45 might take a four-fold increase. Rather than just getting one subject with a really high mark, you could spend this time elsewhere and get say for example four 40s, rather than one 50 and three 30s. Basically, take-home message is to balance out studying your top four subjects.