The math is wrong. its worser then that.
We talking about an sphere here with an increasing volume.
If we take a simple 2 dimension view from earth (circle) and draw an circle over it which represent the flood.
You will see that perimeter is smaller from the inside (earth surface) then the outher perimeter (the flood)
This means if we are talking about a steady rain for 960 hours, that the first seconds are so devestating that you need to think 10 feet per second.
Because the radius is increasing with this mythical rainfall you will need more time to fill every feet after another feet.
DS