I recently added a digital readout to the mill. A few hiccups along the way, but overall the installation went very well. The DRO is a kit from TouchDRO. Ordered the PCB and components, soldered it up and fabricated some mounts for cheap Shars scales.
After calibration, the DRO reads +/- 1.5 thousandths of an inch, which is right about the tolerance of the scales according to the manufacturer.