Always use a high quality standared and use a metric standard for metric reticules and an English standard for English. Trying to convert only induces error. They insure that the standard is accurate by comparing it to known standards. Usually this is not necessary since micrometers measurements are not all that accurate any way. This brings up the issue of relative or absolute measurement.
Relative measurement is used to report percentage differences against some other condition.
An example of relative measurement would be measuring the average size of normal cells and abnormal cells in a specimen and reporting the percentage difference not the measured sizes. An absolute measurement is reporting the actual size of the object based on some standard measurement such as inches or meters. Absolute measurement is used for things such as parts. Industry is interested in the true diameter of drill bits not the variance. Biology based applications use relative size a lot since this can show changes.