Going ETOPS over Greenland where you might have to stay at 14000 to avoid terrain in an decompress situation, and you are looking at about 150 miles to coast around 70N. If I read you right you need spare portable bottles to cover 10% of the pax in this situation beyond the 12 min you get from the generator Also you might be flying over areas where you are no allowed to land with high terrain. Do ETOPS planes in the Atlantic req, the 25 min bottle to allow for Greenland overfly.