Need help with Slide Counter

Hi, I've been using this slide counter example I found, that increases the page number by 1, at the start of every new timeline. Initially, users were not allowed to go backward, and it worked great. Now there will be at least a few slides where the user can go back, and this is messing up the counter. 

Can anyone one suggest an addition to the triggers, so that when the user presses an on-screen previous button, it subtracts 1, instead of adding 1?

I saw an example where this worked, but the user had to use wrapper previous and next button. I need this to work with an onscreen previous button. Also, to advance, the users will be mostly using keyboard short cuts for a simulation. There's only a couple slides per module that have the on screen next and previous buttons.

I've attached a sample with the triggers and variables, and the types of slides I'll be using. Thanks!