These days how I do it is clamp the rod in the mill vise and use my DRO's centerline function. Indicate the back jaw of the vise, then the front jaw, and the computed centerline is the centerline of the rod.
Before I had a DRO I used a long-reach Starrett wiggler with its 0.50" edge finder attachment to indicate one side of the rod, then I computed how far to move over to be on the rod's centerline.
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Law of Logical Argument - Anything is possible if you don't know what you are talking about.
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