Big Dog,
The code does not address the area of classification except in specific locations. It only tells us how to install the wiring after the classification has been determined. This question should be addressed by the design engineer that did the original lab classification. If there is not an engineer involved, I guess I would have to be very conservative and classify an area larger than that specified in the code section that you cited. I say larger because your vent is powered and the ones in 515 are not. The force of the fan may extend the ignitable concentration a greater distance form the vent opening. Then on the other hand, it may also serve to dilute the air stream so that the vapor concentration is below the lower flammable limit making it a non classified area. There are too many variables to answer this question on a forum.
don(resqcapt19)
