Hi, We're offered shaving mirrors by suppliers and sometimes their claims to the magnification are doubtful. I don't wish to upset any customers by giving incorrect information. So is there a practical way in which I can test the magnification myself? i.e. measuring the distance of the bulge inward compared with the diameter of the glass? Please be aware that I do not have a whole host of equipment (no flux capacitor etc) but have some accurate rules and a digital caliper.