Pete,
Dynamite spreadsheet!
That really shines a lot of light on the Protege method. It should be part of the "quickstart" blurb.
One suggested improvement; I really had to study the sheet to see where the 1.001 came in.
One question:
You show 141.85, 141.86 and 141.87 turns. I guess that is skilful eyeballing. It sounds tough to estimate within .82" (82 inch circumference of wheel/100) as the contact point of a tire.
Steve