This might be kinda obvious, but in case someone didn't get it: The whole "It's starting to rain" metaphor is also referring to the running joke that Mustang is useless on rainy days, meaning that he feels powerless after Hughes dies.
How to calculate how much water you can harvest from your roof: Collection area (total square footage of your roof) x x (the number assigned to the collection efficiency of a tile roof) x rainfall (in actual or averaged inches) = gallons/year.
I present to you, the complete comic of the scene from the end of Mark of Athena! Also, 150 hours of my life. I am going to sit in a corner and eat a bo. Long Way Down - Complete Comic - Mark of Athena