Ventilation for 70 cubic foot grow room?

Hey,

I've got grow room of 70 cubic feet/2 cubic meters, I'll use a 400w hps, I'll put 7 Lowryder #2's... How many times the air in the room should be exchanged? In other words, how much should be the CFM(cubic foot per minute) of the fan/fans?

I read from the FAQ that the room volume should be exchanged 3-5 times with above average temperatures, is this the case for 400w hps?
Or maybe someone has grown here with 400w, if so, what kind of ventilation did you use?

I'm trying to get along with minimal but proper amount of ventilation, as on a budget...

Or, how much less ventilation should I use if I would buy Cool Tube(though it's about 100usd)?

Thanks:thumbsup: