Very easy to add in a tab to put a allen screw through to hit the original z stop. Just need to visualize it on your already printed part, put on the printer move it towards the z switch. Once you see where it needs to be, open up sketchup and add in a tab with or without a nut trap reprint and your done. I know its a waste of filament for what you already printed. Your other option is like davidsmake said, add a inductive sensor. OR, you can come up with your own setup.