Cig - your basic calculation is correct but it is considerably more complicated than that. When sizing you have to account for additional drag on the fan. So add this to your calculation: x 1.3 due to the drag of the filter. x 10% for each foot of duct. x 1.5 for any 90 degree elbows. And that doesn't account for any cooling duties that fan must pull. Here is a walk through lifted from the sticky you can find in my signature line on grow room setup. It assumes air exchanged every 4 minutes. 3 - 5 minutes is what you want to aim for:

4x4x6 room = 96 CF. Divided by 4 minutes is 24 CFM required.
24 CFM + 130% ( 10% x 13' of duct) = 55.2 CFM
(55 CFM x 1.5) x 1.5 = 124 CFM (to allow for our two 90 degree bends)
124 CFM x 1.3 (to allow for air velocity lost to the filter) = 161 CFM.

To consider cooling duties add this:

For a 250HPS / multiply by .75
For a 400 / multiply by 1
For a 600 / multiply by 1.3
For a 1000 / multiply by 1.6.