Choose the Zone. Zone 4 is the center of the wall, and Zone 5 is within 4 feet of the corner.
Choose the size of the unit. A 23 square foot specimen would be tested to the 20 square foot pressure. An 18 square foot specimen would be tested to the 10 square foot pressure, etc., as described in the building code.
Choose your roof height and exposure. Roof height is dimension h in the diagram, and is taken from a point between the eve and the peak of the roof. Typically, exposure B is 120 mph or less, and exposure C is 130 mph or more.