Your posting style is still a work in progress.
I did my IANDT Instructor crossover in the spring of '01. My Instructor Trainer was Dick Rutkowski.
My IANTD
Nitrox Manual; Complete Guide to Nitrox Diving (rev 10/96) uses a
T, with Pg on top of the T, Fg in shall we say the "left arm pit" and P in the "right pit."
My IANDT
Enriched Air Nitrox Diver; Student Manual and Workbook (rev 8/97) uses a
Circled T.
My hand written notes from the lectures by the agency founder, author of both above manuals, IT #001, only include one equation;
Pg = Fg x P; no geometric shapes on that page. Perhaps that is because Hyper Dick knew I understood algebra?
My IANDT Sport Diving Instructor Manual, written by Bill Nadeau & Don Townsend, is a 10/99 1st edition. The text with regards to Dalton's Law in the Nitrox section says "using the T illustrated above" and the Illustration is a "circled T" (slide 28) just like the Student Manual and Workbook listed above. Interestingly, I now see that in the Advanced Nitrox section, slide 19 looks to be a typo, with P on top.
Anyway, I no longer have the extra money for multiple Instructor Memberships and have not renewed my IANDT Advanced Nitrox Instructor rating in a few years. You may be right that IANTD
now mandates using a diamond, but that does not mean all experienced IANTD Nitrox divers have heard or care about Dalton's Diamond.