[David Hodgson]

Dale
ADR 16.4 says the wiped area must be 80% of area A, 94% of area B, and 99% of area C. These areas do not include a 26mm perimeter from the edge of the daylight opening, on a Cobra where glass meets frame. This 26mm perimeter can be used to your advantage if you want to use 2 wiper arms

These areas are determined by angles, horizontal and vertical, from the drivers eye position, and vary depending on width of vehicle, but are aproximately as follows;
area A - nearly whole screen, area B - 2 thirds of screen starting at drivers side, and area C the third in front of the driver.

As you may have gathered from comments in this forum, its also dependant on how concerned or fussy your engineer is on the day he checks. You should also arm your self with a copy of the ADR's so you can argue any borderline issues, and not get hoodwinked.