Back instead of "Previous slide" for navigation

When the user clicks the "back" button, I want them to go backward in the linear path of the course, as in one slide prior. I'm noticing that the "previous slide" trigger does not function the way I wanted it to. If you jump around in the course menu, the previous slide could be ANYWHERE and not necessarily the slide immediately prior to the current one in a linear flow. So, the back button works fine when you're navigating forward because "previous" coincides with the slide X-1, where X is the current slide. When you get to the end and you want to navigate backward, it works in reverse..."previous slide" becomes slide X + 1. 

I hope that wasn't overly confusing but the issue is that "previous slide" only takes you to Slide X - 1 if you're navigating forward...as soon as you break the linear flow, it's off. I don't want to hard-code every single slide to point to the slide immediately prior because that's time consuming and should be unnecessary. Ideas?