Though I’m no expert in the field, I’ve worked closely with guys that study wave propagation. They often told me that in urban environments radio waves do some really bizarre things. A nearby chain-link fence or a small body of water can reflect the waves in unusual patterns. I’m guessing that pointing the waves at the towers themselves will give you better results (unless perhaps the building is built of thick concrete, bricks or something similar)... Alas, I think you shall have to do the old trial & error trick to find where you get the best reception.