I don't tend to keep check on the temperature. I watch the puppies - if they're all spaced apart in the whelping box and mum is reluctant to stay in with them then it's too hot but if they're all bunched up, lying on top of each other then it's not warm enough. You want it so they're lying close to one another but not crowding. While mum is in with them, she regulates their temperature. That's a general rule.

A really useful book is The Book of The Bitch by JM Evans - has loads of information in there about everything to do with bitches from normal development right down to whelping and rearing puppies. Might be worth grabbing a copy?

Oh and congrats on the babies