Reticule are made that show a line of a know distance, these then appear on the film. This is the easiest way of showing the size of an object. The most common reticules are linear and net micrometers. The line is either one line or two lines at right angles to each other. These are good for measuring lengths.
Net micrometers are squares divided into regular, smaller squares.
The best way to calibrate a reticule is to use a stage micrometer. This is a micrometer of known length that is placed on the stage and viewed with each objective. Lets say that you have a 10mm linear reticule divided 100 times. Lets calibrate it on a compound microscope. If you are going to use the 10X objective for measuring then swing in the 10X and place the stage micrometer on the stage.