If you wanted to do it with only one button you could get into recall filtering where in most of your scenes the softkey is assigned to scene recall “turn the monitor down” but it your “turn the monitor down” scene it is assigned to recall a “turn the monitor back up again” scene.
That would mean that you couldn’t globally scene safe your softkeys, so you’d have to get your assignments right before you started to save scenes or else you’d need to go into each scene and block it.
If you can use two softkeys you can just assign them to “level down” and “level up”
Chris