Measure the field of view of your new microscope, in other words the width of the magnified image. One can use a plain ruler as shown in the picture to do this. Using the microscope at 20X, measure the distance that can be seen across the ruler. As shown in this picture the field of view at 20 X is about 11 mm. The field of view at 200X is 2.2 mm. If the dial was correct dialing from 20X to 200X would be a factor of 10 and enlarge an 11 mm field to 1.1. In reality, it seem the range is only a factor of 5X, not 10X.