I think 55 is the magic number for most snakes that require brumation. The temps of most NA snakes durring the winter can go as low as 0 degrees but in a subteranian atmosphere I doubt they get below freezing.Snakes experience alot more of harsh reality in nature than in captivity and I think 55 is the safest way to go. Just cool enough to slow the metabolism but not hitting the freezing to death point.
Just my .02
Nate