I'm sorry that your question wasn't answered in a timely manner. Measuring the cervical opening in that manner isn't a valuable manner in which to determine the time of whelping. The cervix will dramatically stretch during stage 1 labor but the degree of opening is too variable to be predictive. Important facts to know include the following:
99% of dogs whelp 63 days after ovulation (the day that the serum progesterone concentration increases above 5 mg/dL).
Labor should begin within 24 hours of her body temperature dropping below 99F (or a serum progesterone concentration less than 3 ng/dL)
If no puppy is born within 4 hours of the temperature readjusting to normal (greater than 100F), dystocia (difficult birth) is diagnosed and her vet should be consulted.
Dystocia is also diagnosed if active stage 2 labor (active contractions) lasts more than 1 hour without producing a pup.
Dystocia is also diagnosed if more than 3 hours pass between pups without active contractions.
Dystocia is also diagnosed if a black, green, or red discharge is present before any pups are born (indicating placental separation and fetal distress) or a pup is lodged in the birth canal.
Please respond with further questions or concerns if you wish.