Having several Hammock Gear Burrow's I generally can get apx. 10 degrees lower then the rated temp. Now that's with wearing all my clothing and tarp pitched in storm mode to prevent any wind from robbing my heat. But I would venture to say you could get to the mid to low thirties with a 40 rated burrow. But YMMV and it just depends on the person and what cloths they are wearing to sleep.
Sometimes I like to hike and think, And sometimes I just like to hike.
Hiking is'ent about waiting for the storm to pass its about learning to hike in the rain.