Unfortunately this setup is designed without any specific Z-endstop position.
I recommend you get an inductive proximity switch: they're cheap, easy to install, very accurate and make the Z-endstop obsolete. You need aluminum bed though.
LJ12A3-4-Z/BX Inductive proximity switch, NPN, normally open (http://www.aliexpress.com/wholesale?SearchText=LJ12A3-4-Z%2FBX). Works without any additional electronics.