i've said it before, and i don't know if it's any useful, but i'll say it again.
i think you should have high aims. anything less is just preventing your development, interest, etc in that subject
however, conversely, i think you should not expect anything too high. this doesn't need to be an absence of confidence - it's just that, do you think that one puny, two digit number totally encapsulates your entire ability in that subject? No - therefore, luck comes into play (how harsh the examiners who read your paper mark, whether the exam is suited toward what you have studied, whether you are in a relatively easy/hard cohort). as such, i think you should prepare yourself for the worst.
also, zig ziglar said it better than me: expect the best. prepare for the worst. enjoy the result.
note how i use 'expectations' differently to him, but in the end i think the meaning is the same, even though the words are completely different