DGP from a given surface

Hi everyone,I have a question about DGP. I want to
assess the DGP at a certain point in my building caused by the façade across the street. What I have done is running the simulation DGP simulation, then setting the
facade as a black body and run it again (this way I account for everything but
the façade). Then I subtracted both results to get the DGP produced by the façade. However, I am
kind of suspecting that DGP is not that simple (since it also considers
factors like contrast) and one cannot operate with it in that way.If so, can someone think of an alternative way to measure glare produced by a certain elemhent?Thanks in advance, Alejandro