Do you have a mentor? Someone (often the breeder of your bitch, sometimes the owner of the stud dog or maybe someone you know within the breed community) who is experienced and up to date? There's really no substitute for having someone on hand, either in person or who you can ring/text any time of night or day, who knows what they are doing and guide you when you first start out breeding. If you don't have anyone then I'd recommend getting in touch with your relevant regional breed club to see if there's anyone who would be willing to help you. I'd definitely count from the first day she was mated, not the last. Better to be prepared early than have the pups arrive unexpectedly. Also, as she may have caught on the first time you don't want to be waiting beyond the due date worked out from the last mating before you start to consider whether she's having any difficulties getting going. Although it's mostly about raising the litter, Puppy Culture does cover the later stages of pregnancy and I'd highly recommend it for information on both subjects.