You also need to consider whether all your readers will use your document in the same way and have the same understanding of your subject. Often, you need to serve a variety of readers. See also pages 109-116 in your text. LJL Seminars offers some effective tips for audience analysis in oral presentations, but you can adapt these techniques to writing. If you're writing a technical report you might like to check out the strategies at this site.
Some writers find it counter-productive to think about their audience early in the process.
I find it varies with the individual task. You'll need to decide for yourself when to start thinking about your audience. My purpose is to persuade my professor to accept my assignment past the due date. Every piece of business writing contains a purpose statement but, of course, you will reword and locate it appropriately as you revise your document. See also pages 106-109 in your text.