I think you found a very good idea !
Let me see if I understood you correctly :
After resizing the lightray drawing, you place the first intersection of the lightray with the circle on the highest points of the building (it's the intersection that is on the "upper left corner" of the lightray drawing ?)
and then you point the second intersection of the lightray with the circle on a separate layer (it's the intersection that is on the "lower right corner" of the lightray drawing ?)

That gives you the dot to connect to obtain the surface of the shadow

Am I correct ?