"Dullage" is a variation on "ullage", the amount by which a container falls short of being full, or where I came across it "an allowance given by the brewery to deal with the need to draw off waste beer from the pumps before serving, or through spillage. "Dullage" can be found in an Urban Dictionary and refers to the vacant space in the head of a particularly stupid person. As in, "dullage is bad thing in co-workers but perhaps a good thing in the blonde at the end of the bar".