One other idea: I've taken my Gossen Ultra-Pro meter, set it to the highest possible film speed (this thing goes to DIN 60 - ISO 800,000) and the largest possible aperture (f/0.7) and taken readings in my darkroom to determine exposure time. So far it has been out of range - more than eight (8) hours.
Although my darkroom LOOKS leaky to my dark-accustomed eyes - I think I am reasonably safe.