First of all, you do not calculate gallons per minute flow unless the flow is pressurized. There is simply no need. In a gravity flow situation the two factors are volume and velocity. The volume calculation is simple in that you use an average rainfall for your area to get the total volume. Example: Length x Width = Sguare area of the roof. Then divide that total by 231 cubic inches which is = to 1 gallon. The total should give you the volume to be drained. Then the size and frequency of the downspouts are calculated. Residential downspouts are typically 2"x3" or 3"x4" and hold a volume per foot of slightly less than 1/2 gallon. A 6" pipe should hold a volume of 1 to 1.2 gallons per foot so you need less of them. Roof Square area divided by 231 cubic inches = Volume (Flow) Volume divided by diameter of pipe (capacity)and number of pipes. I hope this helps.