Here's a handy chart. It tells you based on wattage of light used, how many CFM you need to keep temps within range of ambient.

For example, from the chart, if you wanted to keep temperatures within 2 degrees of ambient with 600w of light, you would need 960 cfm of air exchange. Obviously as the tolerance gets greater, the CFM requirement drops.

There is another formula to figure out true CFM calculating bends and filters and such, and if you want that as well, I'm sure I can produce it.

Iry