I worked with John Blichmann on his article on brewery venting published in BYO.
For a hood with about an 8 sq ft opening area, you need about 400 cfm. There are several factors in designing a hood and blower, so its not cut and dried. Keep the rim of the hood as low as possible while still enabling you to not hit your head or interfere with any stirring. Make sure the hood is at least 6 inches outside the limits of your kettle(s). Try and have an air supply duct feeding outside air into your brewery space as close as possible to the kettle location so that you don't suck out as much of your air-conditioned air.
I cheated in getting my hood to act like it was lower than it actually is. I attached flexible curtains around the edge of my hood that hang lower and don't hurt if your head encounters them.