Tarlanur was able to find this hidden gem from 2009:
http://www.furaffinity.net/view/2354075/

I've only ever seen this concept in one other image:
http://i.imgur.com/YpZZMYn.jpg

I like to think of it as a way to measure how powerful an orgasm is, or how much cum output there is, etc. Kind of like a 5-star rating system, beginning with the most proximal cock. You could easily fit this into one of those gameshow-type TF stories.