My thermostat is set to 45, but inside temp rarely drops below 50. (heat primarily with woodstove, so huge temp variation in front room versus back rooms...when I work it can be 50 by the time I get home.)
All 4 dogs are short hair coated: Boxer crosses, a pitt, etc. 3 of them are elderly.
They prefer it to be about 55+ and below that I see shivering. But they all feel fine getting up and in some cases under the down comforter on the bed. Their dog beds have blankets as well, for when they're lounging, although they all prefer my bed. Or the couch, which also has down blankets on it.
If you have some snuggly place for them and they are USED to it, I think 50 is fine. But if you've recently adopted a dog from a 'non-Mustacian' house used to temps in the 80's they will be miserable and you might need to get them a sweater/coat until they acclimate.
On a related note, I'm opposed to blanketing horses, because even the $400+ specialty fabric blankets will get moisture underneath them, which chafes and can cause fungus and other issues. My horse is unblanketed but can get completely out of the weather in a deep 3-sided shelter.
Her coat naturally fluffs up as thick as a Husky dog's coat.
Because she is acclimated. Dogs don't get quite as fluffy but they do acclimate.