If it's Matt's mackerel sky you're referring to, I just had a look at it. You can add a constant scalar to the seed input of 3D perlin and change the value for different coverage (the billow only gives you the small blobs, the 3D perlin the bigger ones)). Also you can add the whole line into the mask input of a big cloud or PF, and change that seed for overall cloud pattern that give sthe mackerel sky.