what is working distance of loupes, How to measure working distance?

the workig distance is one of the important data for the loupes. the working distace is the distance form the loupes user's eyes to patient mouth or the objects                   

How do I measure my working distance?
working distance is the distance form your eyes to the objects operated when you working as normal postion 
While many things such as height, and personal preference should be considered, the easiest way to figure your working distance is to:

Step 1- measure the distance from between your eyes and the area you want to focus on  (a mouth, an eye, etc), while sitting or standing in the correct ergonomic position, (back straight and arms perpendicular to the floor]. 
Step 2- Subtract 1.5" (the distance from your eye to end of the Loupee

That number is your working distance:
Short       (10"-13")    340mm
Regular    (14"-18")   420mm
Long        (17"-21")    500mm
X Long     (22"-27")   550mm

just need chose the closer one to your real working distance. becaue all the loupes has depth of view. for examples. the loupes working disatnce is 420mm. the real working distance is the range is :380mm-460mm. .