You can't control your pressure gauge, but you can control the cfh.

It depends on how long you want it to take. If you want it to take 30 minutes, I think the calculation is as follows:

4.8 cf x 2 (30 minutes is 1/2 an hour) = 9.6 cfm

If your regulator starts freezing up, which mine does at 10 cfm, I would take it down to 4.8 and let it run for 1 hour.

If I were you, I would get a fuzzy logic controller like the CAP PPM2A. This makes things really accurate for non vented rooms (rooms with AC). If you have a vented room, then I would skip the co2 because your room is too large to properly fill with co2 between exhaust cycles.

If your room isn't vented, how will you determine how often to set your timer?