When you click the side menu item Eye Preference of the Preview tool, this dialog box will appear and you can select the eyedropper densitometer mode. 
One densitometer mode is Exact – this setting will calculate to a double decimal percentage of the exact ink value and produce an accurate display: 
The other mode of the densitometer is Rounded, which means the percentage value will be rounded to the nearest integer and produce a less-accurate display (coarse):