You can make a scale to go around the feed lever that will give a fair indication of depth, but that doesn't help with repeatable accuracy. You can get digital scales for reading tread depth on tyres that cost very little and could be fitted up for an accurate reading. To control the depth without adding complicated fine feed mechanisms you could add a stop screw, ideally with a nice fine thread, and bring the head down onto it when making a cut. This will be fairly rigid, On the next cut adjust the screw a little and bring the head down to the stop again. A friend used this technique on quite large mills and found it worked well. He would set the stop correct for the final depth and add shims, removing them one by one to control the depth of each cut. It was quicker than fiddling about with dials and readouts, and his main concern was accuracy with speed, since he was on piece work.
