Had another play with this file and amde a bit of headway. A couple of suggestions:
The altocumulus layer I was using as a test didn't change much with the different settings I was applying. This turned out to be due to a lower quality setting for the cloud layer. Bumped it up to 1 and things got much better although I ended up turning it into more of a cumuls layer. Dropping the fractals contrast and colour roughness got it back towards it's original frm but with better detail.
Where you have used a surface layer with fractal breakup, I'd tend to stick to settings of coverage 0.5 and fractal breakup 1, using the cloud fractals contrast and coverage adjust as respective substitutes (mainly just to reduce the number of nodes where tweaks are made)
For the cumulus clouds, my experimental duplication of the cloud fractal seems to be getting some ood results. Try duplicating a cloud fractal and set the duplicate as a blending shader for the original fractal, then divide the original cloud fractal's feature and lead-in scales by 100.
I'll post the image when it's done.