It is indeed subjective. There are many things that are subjective, but students are prone to not ultimately making those decisions. This part of the exercise forces you to do so, which then equips you with the confidence going forward to not just pay attention to what's in front of you, but to also make decisions.

One of the general skills we do develop in this course is communication - it's like we're the only ones with access to the reference image, and we have to share that information with someone else, only through what we draw ourselves. That requires us to interpret what we're seeing, and to strip it down into what is most important. That is in itself subjective, and requires you to make decisions as to what is needed, and what isn't. In such circumstances, making a decision - even an inaccurate decision (like in the context of getting the proportions wrong) is vastly more important.