The pH charts posted by DC have been the scientific model for many years. Used throughout the Ag community and and the USDA.
This link might help.
https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_053293.pdf
Thanks for the link, it's a nice chart and it references where it came from, and that manual may describe the process of determining their chart, which would be most interesting. I will look into it. It is also a soil chart..
Now I did not see a chart posted by DC, that you make reference too. I look forward to seeing that chart.
I want to make sure we're all looking at the same material.