Here is a thread started by Joshua L in 2012 concerning the 22,013 T205 cards he researched, and the various percentages for the backs. It is interesting to note the very large difference in percentage for the Piedmont 25 cards in this 2012 posting, and the SGC article. I assume the SGC article was strictly involving the SGC T205 percentages...?
Also Joshua broke down backs a little more thoroughly, for example percentages for Broadleaf in both Black and Green ink colors.
Here is the link to that thread:
https://www.net54baseball.com/showthread.php?t=157949
Post # 40 on this thread contains a table another member created that is a little easier to see.
Brian