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 six feet long since the unit is a standard 8 ft wide. Each of those chalked-off areas will take one gallon. So, when you are done applying in that area, you should be done with a gallon of the product.