Swales: The Permaculture Game now has a roof * rain calculator and visualization map. It works in metric and imperial, although the
water tanks are in round imperial numbers.
(100,000 gallon, 50,000 gallon, 20,000 gallon, 10,000 gallon, 5,000 gallon, 1,000 gallon and 500 gallon).
The intention is twofold, make it easy to calculate how much rain you could be collecting, and get a visualization of that. I've already demoed it privately with some supporters, and it is interesting how seeing the results in 3D (and the numbers as well) gives them a different perspective of what is going on with their property.
Please have a look. I believe that as this matures it is going to become an invaluable tool for all of us when designing or managing properties.