Hey, i also ran into the problem, cause of a large mesh. I played around with the Decal Material. Idk why but if you hit D3D11Tessellation Mode to Flat Tessellation it works for me. Then i put the WPO Parameter to -0.01 do avoid the z-fighting, when i get closer to mesh.
Iam not sure, if this is correct, but at least it works.
Please correct me if, if its something i should not do 
i did not test it, but if you subdivided your decal mesh in maya/max or what ever, it also could work.
