Hi Ian,
i just saw your mails and i assume from your links that you are also located in Canberra. I have an OSPI connected and started to look into the Zimmerman method (got it two weeks ago). You seem to have adjusted the Zimmerman values and it works for you.
My program scheduled in the morning at sunrise does not really work. are you willing to share your values that would be great. i will also have a look if we could use bom data from your link, that would be even better.
btw. i am in Evatt.
Cheers, Bernd