Biology based applications use relative size a lot since this can show changes. Once a microscope is calibrated it is calibrated for life unless you change the configuration. Frequent recalibration of a reticule is a waste of time and money. Some medical inspection persons require this and it is a total crock. If accuracy is a real concern you should use a video based measuring system or a filar micrometer.
Filar micrometers have a moving line visible in an eyepiece that is driven by a micrometer drum.
The distance is read of the micrometer drum or of a digital display. These work very well if the specimen is rectilinear. They are used a lot in industry. . You have to move things around to get the measurement. This leads to interpalation errors.