There is no easy answer.
If the temps are too high, you risk cooking your plant. If they are too low, you risk killing the new growth.
New growth is very tender. It is temperature sensitive, probably damaged even above freezing, which is why at 40F figure is given, most likely. Alos, ne growth is easy to damage by touching it, by suchthings as throwing a blanket or tarp over it to protect from low temperatures.
if you do a search for "fig shuffle" you will find the same issues arise when storing the trees indoors. Sometimes they can be outside, getting a start on the season, and sometimes they have to return to a protected environment when weather turns cold again. It is a game and a pain, trying to balance things, particularly in the Spring when temperatures fluctuate dramatically, sometimes.