Beware. Stanford is notoriously harsh in REA. They defer much less than other schools - choosing to outright reject instead.
You are correct that the 10% figure is a bit misleading because of the applicant pool (legacy, athletics, etc). If there are some other schools that are close on your list, maybe you would want to go with them instead. At least at the others, you typically get two kicks at the can (early, then regular) if you need them.