low humidity

So I have a room that is 9ftx3.5ftx6ft , from calculations I've seen you multiple these together and then multiple that answer by 1.2 (20% for carbon slumber loss) which gives me 226cfm, I have a 300 cfm 6" fan, my filter and fan are mounted to ceiling above light together then a 6" duct runs...

I've never had an issue with drying my herbs... but I've never done it where the relative humidity is between 30 and 40% on a daily basis. With the humidity this low, if I use the method I would normally use at the beach (hang 3 days, then cure in jars/ziplocks), it will be dry and brittle...