I think you'll have to use your judgement to determine the relationship between the reduction in incident beam irradiance to the array's electrical output.
You may want to read the reference cited in the PVWatts V5 reference manual with the passage you originally quoted in your first message. That report describes an NREL study that shows the reduction in output for different shade conditions:
Deline, C.; Meydbray, J.; Donovan, M.; Forrest, J.; Photovoltaic Shading Testbed for Module-level Power Electronics. NREL Technical Report NREL/TP-5200-54876, May 2012. (
PDF 1.2 MB
)
Best regards,
Paul.