Calculation of final magnification when using a CCD camera
Assuming that the FOV that is captured by the CCD on the screen is 100µm wide as determined by the micrometer, and you are viewing the 920 pixel wide image on a standard 96 ppi monitor, the zoom would be approximately 2434 times. 

$$\frac{920p}{96p/in}/100µm \approx 2434$$

Additionally, you should ensure that the minimum resolution on the micrometer is at least twice as large as each pixel on your microscope CCD. 

The reason that the pixel size must be at least 2 times smaller than the minimum resolution present in the image is due to the Nyquist-Shannon sampling theorem. 

This being said, all microscopes (and arguably all measurement devices) should be calibrated against a standard. This paper explains the usage of a standard to calibrate confocal microscopes. 



A similar technique used to calibrate normal optical microscopes using a stage micrometer standard can be used if you do not possess such a standard. First calibrate the normal microscope, then image an identical sample using both microscopes in order to obtain the size of the field of view.