Generally TNF goes by the EN Lower Limit for their bag ratings, so this should be a pretty solid estimate.

based on the en limits for the tundra -20 bag, tnf appears to be overrating their bags by about 15-20 degrees.

I asked the same question to TNF customer service. They told me that "The North Face has tested and labeled all of our sleeping bags -- globally -- according to European Standard EN 13537 since Spring 2010".