The length of gestation is based on 62/63 days from ovulation. If you don't do progesterone or lh surge testing the exact day of ovulation cannot be determined. Since a fertile mating can take place approx. 5 days before or 5 days after ovulation this gives the appearance that gestations can vary in length.
The temperature method (
Dog Breeding and Canine Reproduction by Debbie Jensen. Dog Breeding and Whelping guide for dog breeders.) always worked for me. I looked for the spike followed by the drop (right at 98 or below) and labor begins within 24 hours. Also, mine always refused food the day of labor.