I believe that aiming at a certain length is generally a bad idea, and it is not good for a journal to give such a narrow window. Articles or dissertations written with a strong intent to reach or not to exceed a certain length generally do not read good. It is best to write articles in the natural way, and the natural way is to focus on what you want to say. Furthermore, an intent to meet a certain length requirement is often clearly felt by too succinct explanations or, on the contrary, large pieces of information of little value. If I were you, I would seriously reconsider the choice of the journal.
But if you really want or need to submit your work to that journal, then I would advise to aim at the middle of the window, for the following reason: if you happen to write somewhat shorter or longer than planned, you will still be inside the range. Also, an article of a length around the middle of the allowed range won't make anyone think that the author may have struggled to meet the length requirements by expanding or cutting the article.