Parametric Block Stretch Direction

I'm trying to create a simple parametric block that can stretch. It works, but I want it to stretch to the left and if I input a positive dimension while stretching it goes to the right. Does anyone know how to make it work so I can stretch it to the left with a positive input? I'm trying to make this simple for other users to just be able to input the width they want it to be. I know this can be done in the parameters tab, but I would like to make it work at the dynamic input of the grip point. See short video of how it's currently working.