I have a six inch, mechanics steel rule.
I scan my stamps the toggle the images to isolate the small changes
then pencil on the back as templates.
The steel rule works find, using the loupe in the right hand
as one measures.
The ability to scan, save, and crop images
then "toggle" them is an extremely useful method for measuring
and comparing stamps, esp for "flyspecking"
It is understood, new collectors may not have the finance to support this..