Most of the time, the temperature will drop to 98 and then you can estimate 12-24 hours before birth. You may want to check out
Dog Breeding and Canine Reproduction by Debbie Jensen. Dog Breeding and Whelping guide for dog breeders. which could give you a lot of information. You may also want to post in the breeder section. We have a lot of wonderful breeders that can give you more detailed information.