This is the easy way, determine a distance from the ways to center and then use a ruler to measure the tool, I have done this for years without ill effect.
If one is truly determined to have the tool on center (parting to 0 for example) just run it across the face of a part and adjust it until the nub disappears, this approach is fool proof
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.