Thanks for you comments RJ. Makes total sense to me. That Edge bar looks very useful, thanks for link. Setting up my DTI wasn't too difficult & I learned some new things I probably should have paid better attention to. If I set the DTI ball to a slightly different position within quill ID, it was usually pretty close but could deviate 0.0005" depending on the wind I was similarly concerned about repeatability especially if the tailstock was further away & my Nooga arm was hanging way out, maybe influenced by gravity? The nice thing about that bar method is it uses centers which are more engaged & averaging within the socket & also over longer distance so less prone to picking up any local deviations