Type a move command in the console window for the value you want to go down. For example 0 your machine, then send a -.5 command to move the Z down by what you want (in this case 0.5) and then hit the Zero Z button. Then your “0” will be whatever you moved it lower. I am certain you could make a macro up and store it as well (I am not an expert here) that can zero the machine, drop it by whatever and then re-zero it.