I made an aluminum gauge rod that gives me the centre height as a fixed gauge. I can feel within .002 quite easily to make sure the tool is on centre.I usually achieve closer to .001.
I measured using an electronic vernier the height from the saddle top to the top of a known gauge block in the chuck, then the height from the flatway to the saddle, and subtracted 1/2 the diameter of the gauge block.
That established the length of my aluminum setup rod.