Use single slider to scale varying x,y,z values

I have a PG (here) with a set of buttons that allows you to control a boxes scaling with a single slider (based on what button is pushed). My Height, Width, and Depth work well. The slider is set to the previous position and the user can adjust from there.

I can’t seem to figure out a way to get the single slider to increment the global values. I’m having trouble determining move left (subtract from) or move right (add to). Ideally I’d just use a +/- buttons but for consistency with the UI I’d like to use a slider.

If you adjust the height and width so you create a shape with unique x,y,z scaling values, I’d like the global button to maintain the ratios of each x,y,z and increment them. I’ve tried a ton. The closest I get still doesn’t shrink when moving left (until you move to a value below 1).

Any direction is greatly appreciated. Thanks!

Something like this would do:

Basically keeping track of the global value to understand between each step if you go up or down.

That’s it! Thanks. Keeping track of the global value / step was what I had missed.