With DS my water broke and he was born the next day - at 36 plus 6 based on the ultrasound, 37 plus 2 based on LMP. I have long cycles and he was tiny, so the early us was more accurate than the LMP date.
So, I would be happy to go beyond 40 weeks. I know my dates for sure this time as I charted, and would start NSTs at 41 weeks probably and start natural methods at that point. The birthing center cannot take mamas beyond 42 weeks. But I doubt Iwill ever go that far, only two babies in my entire family were born past 40 weeks (and only 2 days after due date).