I thought I read somewhere that standard screens filter out about 30%, but I don't have a reference right now. I'll see if I can find it.
My somewhat educated guess is that the amount of UVB filtered out is a function of the total area of the screening material within a given area. The smaller the holes between the strands of material, the more filtration takes place unless the material diameter also decreases.
In other words, if you have 8 sq. ft. of area, and the screening material takes up 30% of that area, you lose 30% of the available UVB.