Rule of thumb - match the cubic feet per minute (CFM) rating of the inline fan to the cubic feet of your garden area. Use the formula L x W x H to find the cubic feet of your garden area, that is the fan CFM rating you want approximately. If you are using a filter you want the filter CFM rating to match the fan CFM rating.
There fore you would need something along the lines of 960cfm?
You also want to add 10% to your overall CFM for your pressure drop! Especially when there is any filtering involved!