This site explains the temperature drop well and what you're looking for:
http://www.debbiejensen.com/temp_chart.htmlEven got it in old money :-O
Without converting (I'll leave you to do that!) I find when my girls drop into the 36sC then that's an indicator they'll whelp within 24 hours. But, it drops for a short time before rising again, so you may well miss it! Good luck ... puppy watching much more exciting than Christmas :-D