Did you see her bred? Icky tail at milking time or the buck talking to her? without anything more specific you may have to go off her first day of exposure. A blood test might be a good idea if you think there is a chance that she's not bred yet.
When I pen breed I'm very careful to keep really good notes on anything I see. I've had good luck with due dates writing down discharge and gross butts.
I would probably go with the first day of exposure and err on the side of caution. Which would have her due around January 1, 2009. In turn this would mean that you would want to have her dried up on November 1.
A blood test is a good option. I think they are accurate after 26 days pregnant. If you were to do the blood test, you would only know that she is at least 26 days along. You would not have a due date, but you would have confirmation.