28/13 is 1 day off per 2.15 worked. thus 365/(2.15+1)=115.87 periods a year
115.87/12 =9.6 off a month averaged over a year. Your currently getting 8 on the average....so an extra 1.6 per month off with your family.
Legit days off could well be down route, as they were advertised in the last freighter version.
747F are still flying, and I would be surprised if they were terminated since they carry odd sized freight that the 777F cant carry. If they have been terminated I wonder what they were previously then?