For EPDM roofs we highly suggest applying a flash coat (a very light coat) approx. 200 sq ft per gallon resulting in 6-8 Mil. Then once dry the final coat at 50 sq ft per gallon resulting in 20 wet mi.
You can chalk off you roof into sections 6 feet long since the unit is a standard 8 ft wide. Each of those chalked off areas will take a gallons. So when you are done applying in that area you should be done with a gallon of the product.