Modified garyacrowellsr's clip to hold a Pittsburg 1" Travel Machinist's Dial Indicator, item 623, as sold by Harbor Freight (http://www.harborfreight.com/1-inch-travel-machinists-dial-indicator-623.html). It's a very cheap analog device designed precisely for this purpose - measuring the distance from a mobile mount point to a bed to level it, with a "roller ball" tip so that it moves smoothly over the bed surface. The clip holds the gauge in a vertical position so that it's easy to read. The gauge mounts using the lug on the back of the dial, which is easier to assemble than force fitting the dial's shaft as the other gauge mounts on Thingiverse do. And I think that by attaching to the carriage it is more consistent with the movement of the extruders.
Works very well with the new Sailfish leveling script.