If the observer is at 800-850 feet, with an intervening hill between him and the target buildings of 650 feet, and we know the distance between them, then simple school-level trig will tell us the downward angle from observer to intervening hill, and by extension from that, the height H at which that sightline should meet the buildings - IF the underlying plane and reference level for all the heights is perfectly flat.

Like this;

Storm, I invite you do the calcs for us ...