I may have missed something (I have been doing a lot of skimming and skipping).
I didn't think the percentage break down had anything to do with the genotype. I was thinking this was something like the Araucana color cards. A way to describe a the phenotype ( 20 different shades of blue/green eggs even though all of the birds are assumed to be O/O in the genotype and similarly 8 shades of cream even though all the birds are assumed to be ig/ig) .
I assumed that you could say that the color of you flocks was in the 10-25% range to describe the shades of cream you are seeing, or you could say that you flock was in the 33-45% range.
It didn't look like this chart was set up to describe the Ig/ig or Ig/Ig birds since there is a gap from 50%-100% which all the gold birds would probably fall into.
I am still a long way off from talking about shades of Cream, but in 5-10 years when I get to that point I though this type of chart would be intended for that purpose.
I have noticed red enhanced birds (cinnamon in the chick down, red on the throat of the hens, ginger on the crest of hens and cockerels, ginger barring on the saddle feathers of cockerels, etc. ) and birds with out red enhancement. A cream bird with red enhancement may look more like 50% where a cream bird with out red enhancement may look more like 20% and a cream bird with additional dilutors of gold may look like the 0%. All of these are just examples. I really have no test results of observations to be able to determine what genotype makes each of the colors on the chart above. Fortunately I don't have to know the genetic formula. I just need to know how my flock breeds and what their tendencies are in the offspring to get the colors I want.
I have not seen this chart but I understand your point in using it. I guess my concern is that the phenotype coloring that it may be describing may have little to do with the actual cream gene and what it does is lump the different melanizers and diluters into one big pile with the cream. The descriptive nomenclature as a being a % of cream would be incorrect as it's not specifically speaking to the cream gene but more so to autosomal, pheomelanic or eumelanic issues. So rather than a description of cream it seems to be more so the melanizing of the plumage its describing, and those are still an issue with many birds here, mine included. Cream is cream genetically so I find that it just confusing the issue, especially for newer breeder when this idea of a % of cream is used. If the bird is not double cream, regardless of melanizers it is not a Cream Legbar.
Parts of the plumage are more susceptible to enhancement than others but others not so much, for example major portions of the hackle will definitely denote whether the bird is cream or not regardless of enhancers while the areas at the top of the head and bordering the neck and back may show influence.