With Joe, I am constantly monitoring him during the wind down. If we have a few warm days the countdown restarts for him - you know I have a lamp outdoors for him, but when the time comes for hibernation he won't eat even with basking under the lamp.
He seems to know when frost is coming and it's not unusual for me to put him down and the frost happen that night.
With modern technology, I monitor accuweather in the run up to hibernation - looking for a maximum of 9C consistently before I'll hibernate him.
He's double boxed in our garage and I have a min max thermometer in the box with him. The 'outdoor' wireless probe is in his box and I have the indoor part so I can read what's going on.
I have been known on very rare occasions to wrap a couple of picnic box ice packs in a towel and put them in the outer box to get me through a freak warm week in January. But I try not to do it as I'd rather get him up if the weather turns warm.
Even during our winter cold spells, our daytime high is seldom lower than about 12-13C here. We get down to 0C, or even a little lower, on occasion over night, but winter days are usually around 21-22C and sometimes as high as 32C for days or weeks at a time.
This is what I'm referring to when I say that its consistently too warm here for above ground hibernation, and its consistently inconsistent.