I messaged the Italian and German consulates with questions about the Schengen Zone and I had a reply from Italy.
My understanding had been incorrect. I had thought that you get 90 days in 180 days – in the sense that two clocks start upon entering a Schengen country: a 90 day clock and a 180 day clock. When you leave Schengen the 90 day clock stops but the 180 day clock keeps going. When those 180 days pass you get a fresh 90 days in the next 180. But no: its 90 days in the past 180 days. So today replaces 181 days ago and tomorrow will replace 180 days ago, etc.
Italy sent the link to a calculator and a detailed explanation. All of this I shared on a couple of bicycle touring Facebook groups. There I learned that due to agreements which pre-date Schengen Australians can get extra time in Germany and Denmark (I think) and someone shared a link to a Wikipedia page outlining all the visa requirements applicable to Australians everywhere.
In other news, having fully caught up my bookkeeping I’m amazed to find I may have nearly twice as much cash on hand when I pedal away than I had set as my necessary minimum for the trip. Crazy.