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Radius on small surface.

Hello All,
I'm trying to get a radius on a small surface (about 4 mm wide). I've measured and remeasured the same radius and I'm getting numbers that aren't exactly consistent. (13.155 mm, 12.952 mm, and 12.843 mm). The nominal is 12.40 mm. Is there anything anyone can suggest to get more consistent results on such a small measuring area?