You know those printable measuring charts? You download a PDF and print it on a regular sheet of paper, and cut out the included measuring tape. That way you can get a way to measure centimeter length of any object as long as you have a printer!
I need a tape like this, but not to measure centimeters - rather to measure a diopter unit (I'll provide a calculator to convert centimeters to diopters, it's quite simple).
Point is to make it simple and yet visually distinctive and enjoyable to use.