I have several Bowers Boremikes, on one of them the zero is off by 0.0003". I bought these used and have no manuals. They are themechanical type with the plastic window over the micrometer barrel, this one covers the 3/4" to 1" range. When I check it against the ring gage it's reading high by 0.0003" so I'm guessing there must be some way of rotating the barrel or the spindle to compensate, but It does not seem obvious what to adjust. Anyone play with one of these?