Summary

An adjustable endstop holder for either opto or mechanical endstops.
The endstop holder is static, as I found that most of the strategies for adjusting this introduced some wobble. The adjustment mechanism is attached to the X-carriage end using one of the screws which hold it together (or drilling a hole if you don't have any handily located screws).
The screw adjusts the height of the endstop flag using my 'decimal' thumbscrew - (originally from my bed levelling kit - http://www.thingiverse.com/thing:69074). And requires a spring (a printed option is in sources). The flag also requires a fork to cradle the Z-smooth rod to stop it from rotating.
Et voilà an endstop easily adjusted to 0.025mm (this is 1/20th of a turn of the thumbwheel assuming you use an m3 screw) and more accurate if you care to be more careful with the thumbscrew.

Unfortunately you'll have to use a little imagination as my sanginolulu just died, so the adjustment isn't completed (as you can see in the photo the metal 'place-holder' at the top) . Replace the metal bit with a shape to fit your setup which cradles the z-rod, and the lower nut should be replaced with the thumbwheel.