my bitch had two slip matings and then tied four times over the next four days, when would you take her first due date from

My experience is not great but I would think the first slip mating as bitches have produced from one slip mating so that would be the first date to count from, would also depend on if she was ovulating at the time.

I would tend to say that often slip matings mean the bitch isn't quite ready adn because she mated for up to 6 days after that I woudl tend to bear the first slip in mind but count from the first proper mating.

If you had premate tests done then it is easier to say:

See here: http://www.ivis.org/advances/Concannon/concannon/ivis.pdf

Fertilization and Gestation Length
The reason why gestation length in dogs is relatively consistent when measured from the day of ovulation but
highly variable when measured from the day of breeding is partly understood. In the vast majority of bitches
parturition occurs 64, 65 or 66 days after the ovulatory surge in LH [1]. The latter represents the acute release
of LH from the pituitary in late proestrus or early estrus. The LH surge triggers the event of ovulation. Since
the day of the LH surge can be measured or estimated with reasonable accuracy, timing events from that day,
and using it as the reference point (Day 0) can be helpful. A 64 - 66 day gestation length measured from the
LH surge to parturition is the same as a 62, 63 or 64 day interval between ovulation and parturition, since
ovulation has been estimated to occur 2 days after the surge in LH [4].