Hi Michael, is this for something like a newsletter? If so, you might consider having two fields... One that is specifically for the form, and one which is actually used to know who will receive the newsletter. Reason for this being: a form where someone can subscribe should not be counted as an opt-out if they don't check the box, and boolean fields can only be true or false.
What I have is newsletter form field, and newsletter field. If someone checks newsletter form field, then newsletter field is changed to true. If they leave newsletter form field unchecked, then newsletter field doesn't change. If they opt-out via our unsubscribe page, then the newsletter field changes to false.
This might be good practice for all forms, CASL related or not, because if someone unchecks a box due to thinking that they already receive the newsletter, then that uncheck can potentially be removing them from the newsletter.