In reality elastomeric materials should only cover approx 150-200 sq ft per gal. Should also be back rolled behind the sprayer.
All paints include instructions on the container which also specify typical coverage. Typical elastomeric paints go on think (~15 mils), and will only cover 120 sf/gal. Most also require 2 coats for a waterproof seal, which will require twice as much paint (bringing coverage down to just 60 sf/gal for 2 coats).