One thing I was thinking is that you could easily measure whether you have enough fans or not. Get a couple of inexpensive indoor/outdoor thermometers. (Radio Shack sells them). Put one temperature probe in your hood, but not directly under your MH. Put another one someone in the room, but reasonably close by. If the two temperatures are within a couple of degrees of each other, then you don't need any more fans because the fans are equalising the temperature. The thermometers are only accurate to +/- 2 degrees, in my experience, so that is about as close as you are going to get.

I have never tried this myself, but it should work. Personally, I would not want to have more fans than I really needed. That's just extra noise.