I would not go with anything rated below the CFM of the fan. You may dial it back to 55 CFM but you may find that that does not meet your needs and that you have to run it at 75 CFM in which case the filter will not be able to keep up. The 38 special might be overkill. In my case I can not afford failure. I periodically have a room full of lawyers above my grow room so I'm spending a little extra cash for a little piece of mind!