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. Focus on the stage micrometer and center it using the stage. Use the eyepiece reticule and measure a distance on the stage micrometer. Lets say we measure from the .2 to the .3 on the stage micrometer. The measurement we get is 1mm, we counted 10 division when we measured, so each division of the eyepiece reticule measures .01 mm.