I'm still learning; but, I've had luck with the following:
Main
• Cumulus. 3D Volumetric
• Cloud altitude 2115 (or, to suit)
• Cloud depth 1024 (or thicker)
• Edge sharpness 100
• Cloud density 0.00225 (increase for more clouds)
• Coverage adjust 0
• Cloud colour 0.35
• Scattering colour 0.5
Lighting
• Light propagation 2
• Light propagation mix 0.125
• Fake internal scattering 0.25
• Enviro light 1
• Enviro light tint 1
Tweaks
• Ambient 0
• Fake dark power 0.9875
• Fake dark sharpness 10
• 'check' improved lighting model
• Base wispiness 0
• Base softness 0
• Coverage gamma 0.9
Quality
• Rendering model 3D (volumetric)
• Quality 6.88104
• Number of samples 438
• Sample jitter 1
Pattern
• Seed number 42525
• Feature scale 500
• Lead in scale 20000
• Smallest scale 10
• Noise octaves 13
Experimentation is the only way you're going to find the results you are looking for.