To determine the width of a Phosphor bar can sometimes be tricky. The correct method is to determine the measurement of the gap between the bars in mm ( not the band/s).
Subtract the distance measured from 21 mm, which is the normal width. ( vertical Machins only with 2 bands or one side band). I do have a distance reference for most Machins written in a text book.
To continue; IE: If your measurement of the gap is 12mm subtract this from 21mm which will = 9mm, divide by two and we have 2 x 4.5 mm bands.
There are many loupes on the market. I myself I have two that magnify and measure with accuracy. The first one is a German made "Meetloep" x 10 with adjustable scale up to 10mm with 0.01 & 0.05 divisions, this was recommended to me years ago by James Skinner.
I choose another later, for general use, this is a "Ted Pallar" x 10. Pallar inc have a comprehensive list of magnifiers and are well know for quality and resilience.
The image of the print, perforations on all stamps and in particular the phosphor bars (with a UV lamp) are easily visualized and can be accurately measured with this handy little tool.
This one (left) has served me well as it is now 5 years old.
Summing it up : A precision Magnifier of fine quality, with a removable coated achromatic lens. It has high resolving power with a wide visual field and is provided with measuring scale of 20mm in length, with 0.01 & 0.005" divisions. Perfect use on a desk at the home or office, its small (eyepiece: 18mm. The field of view ( 36mm) has a scale diameter of 26mm. The height is 44mm.
The weight is only 56g so it is portable, which is great for taking to shows or exhibitions, it also comes with a case, lens cap and cloth .