I know 55 deg is spouted as the default, but it's so broad it's like saying "Oh it's below freezing, so it will snow". I think solar insolation isn't given enough credit here. At 4" depth I might be 50 degree, but at the 1" depth I could be at 60 or 65, especially given the last few Februarys and how warm they've been.
I just go by feel for my yard. I have some pansies in the beds every fall that I let over winter. When I start seeing them re-green and the daffodils growing then I know.
The Fescue is far more wind burnt and browned out this year than the last few years, and the Zoysia is the most golden tan i've ever seen it.