
I would go from the ovulation date.
With our present litter, we didn't have her progesterone tested, but instead relied on my male to let us know she was ready (Friday evening the first signs)... and we went to the stud on the Sunday. One mating later, she produced a good sized litter early on Saturday morning exactly 63 days after ovulation (presuming the start of her ovulating was on the Friday evening, she actually went into the first stages of labour on the Friday evening, whelped pups early Saturday morning).
Can be very hard to calculate if you go on matings, especially if you have two or more... although I understand this is the traditional way of calculating whelping date.