Thanks for the tips, I'll be keeping a close eye on the things you two mentioned.

However, I've actually decided to start again from scratch - my first runthrough of the tutorial took a while, and it's easier for me to try again with a new map than attempt the process again from halfway through. It'll also be a 2100x2100 map, so I won't need to fiddle with any settings like I had to with the other one (which also confused me a bit; I wasn't sure where to add the extra Fill Basin and Noise step that you mention using for larger maps).

I've just completed the land cloud isolation step, and I've run into something that caused me some trouble before, namely that my land clouds are a lot darker than yours. Is this something I should just correct later when airbrushing, or is there a setting I should have changed to keep this from happening?

Click image for larger version. 

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As you can see, there are parts of the landmass that are so dark they're indistinguishable from the oceans.