Actually it is that simple. -8 could carry 295,000 Lbs of revenue payload. To make the math work you would have to fly them 425+ hours per month what would mean 4 round trips from Asia to either West Coast or East Coast with a fuel stop in ANC.
When Atlas first bought the -400's they realized savings in MX, dispatch reliabity, the ability to fly non-stops ( 10 hour stage length) with acceptable payloads and 500 gallons per hour in fuel savings. Back then fuel was cheap and saving 500 gallons did not mean all that much. The -8 will burn 370 gallons less than an -400 but based on a payload that is 55,000 REVENUE pounds higher.
At $2.20 per gallon and a full load the -8 is more expensive to operate per trip bases, but cheaper per unit of weight/hour (either ton, kilo or pound). So if you are a shipper or freight forwarder and you believe you have 295,000 lbs of cargo every time the plane leaves the ground the -8 is a money maker. As I said, that simple.......