Hey Mikey,
Either one can be used, the link you had connected to a dial indicator, so I assumed he was also interested in a DI. A DI will read true distance, a DTI is relative. I use both, but for headstock alignment and setting up stock in the 4J I use a DI, I use a DTI for close in work near the chuck or inside stock alignment. Most of my work requires tolerances of better than 0.001" so I use my Starrett 25-611 most of the time and it is much easier to read than my Interapid which is a 0.0005" model. The nice thing on the Interpid DTI (also Compac, Tesa, newer B&S seem to be the same) is that they have a several revolution range measurement, so on their 0.0001" the range is 0.024". I would look for NOS or lightly used, sometimes you can find one at a decent price, but seems less so these days.
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