zelor was kind of vague on how far humidity should be pushed inland
Yes, that is not the strongest point of the tutorial.

n suggesting using the paint-bucket tool
I usually just paint the border unless it's necessary to paint the whole thing by hand (when it's small, for example)
and fill the rest with the bucket.
I haven't mentioned it because I guess it's like you said, there are some time-saving tricks that you learn overtime.