Njen, is this what your after? I just played around with Wills file (thanks Will). Precise placement of clouds is not really possible, since they're randomly generated by the seed value. I changed this 'till I got something roughly right then as Will said, adjusted the Density values in the Density fractal (Cloud fractal shader3). Don't know what ticking the clamp low box does, but I ticked it anyway!
Kevin.