I dunno' Greg. For b&w all I use is a single layer of Roscoe #19 Fire between glass sheets without any diffusion. The Roscoe spectral transmission chart says this should be sufficient. And for both my CD and pre-flashed fogging tests of Ilford MGIV, it is.
This filter is located in the lower filter position. The vanes contain only sheets of black mount board. That allows me to adjust down to total darkness for easel framing and focusing.