Z -ve Limit Switch

I'm setting up Limit Switches on my Proxxon Micro Mill. I've noticed in
wiring diagrams collected off the net that some people don't set a limit
on -ve Z (I think that's down towards the table?). I would like to fit an
adjustable switch for this limit so that I can set it when I am proposing to
machine perilously close to the table surface.

What do other people do?

Thanks
Steve Tompson
Bearsted, England