I know DE is good for the girls to help with pests and disease, but how's the best way to administer it? I'm thinking about providing a tub with dirt/sand mixed w/DE so they can dust themselves. Or maybe leave the area under the coop (4X8) in dirt, add DE, and let them use that as a dusting area. How much dusting area do they need and how often do I add DE? I'll have 12 hens using the dust bath.