Argggh -- you're making me think. It hurts, it hurts. You cruel bastard ...

A 100cfm fan will vent up to a 10'x10' room. So I gree with SyKo's numbers. Bulb temp, test with your tongue ... if it sticks, it's over 100 degrees.

Better to go a few CFM over than be under. Keep your ballast outside the grow area. I think I double up on the CFM to make sure it stays cool. You can use a regular bathroom fan which you can pick up for less than $15.00, along with some flex tube