Finding your audience is an art, not a science. You aim for a general ballpark, but there comes a point where the ball is going to fall where it falls. Consider what you're writing and start to daydream about who you are talking to. Remember that each person who reads your work has a one on one relationship to it. So while you might be speaking to many people, you're actually speaking to one at a time. Take time to envision them one at a time. Post a picture of who you're writing for at your desk.
Once your material is ready for viewing, put together an excerpt or the full piece and deliver it to free or paid readers with a survey of questions, some multiple choice, some fill in the blank. This process will tell you who your real audience is. For a free copy of a sample standard survey, write me at firstname.lastname@example.org!