How do you read 0.126millimeter's on a ruler?

You generally cannot read 0.126 mm on a ruler. You would be lucky to be able to resolve 0.2 or possibly 0.1, but not 0.126. Even if you could, and its true that 0.126 is about 1/8, the standard for accuracy in a measuring device is ± one half of a minor division - that is ± 0.5 mm, so a reading of 0.126 mm is meaningless, unless you had a much finer lined ruler and a microscope.