Knowing only the supply airflow, extract airflow and room volume is not enough.
Firstly, the room volume has no effect on the static pressure that will be maintained once equilibrium is reached. It just has an effect on how long it takes to reach equilibrium once the fans have been turned on.
The difference between the supply airflow and the extract airflow is important, and it would seem that you either know this already or you can find it out. This difference is the amount of air that must be taking a different route, i.e. through fabric leakage (gaps at doors, etc, for instance).
What you also need to know is how "leaky" the room is. For instance, if the difference between supply and extract is, say, 250cfm, and the only leakage you have is around a "standard" double door, then the typical room static pressure would be, perhaps, on tenth of an inch of water.
You therefore need to estimate (I won't say "guess"!!) the room leakage. You will find that there are some tables published that show the pressure drop across different leaky fabric elements at different leakage rates.
Regards,
Brian
