<p>I think you should start by taking a practice test in Chemistry and Biology and see how you do. Buy prep books, which is not a big investment of money, to help you get a sense of how well prepared you are. This is a good idea anyway since you are not taking the test immediately after the course finishes, so the prep books will help you refresh your memory. Sign up for the October 11 testing date. Someone correct me if I’m wrong, but I think you can walk in and take a different subject test than the one you signed up for. If your intended schools don’t require all scores, you could take both Chem and Bio and see which one you do better at. </p>