Firstly: Great photo! 
I recently wrote a comment in another thread with some quick tips for using the retouch module (instead of spot removal) @
Excerpt of the important points:
shift-click on the image to set a source point (optional, but useful to avoid adjusting it later)
control-click on an icon in the retouch module to stay in that mode , so you can continually retouch the image (such as dealing with multiple dust spots in the same go)
use your scroll wheel to adjust the size ; hold down shift and scroll to adjust the feathering
you can turn on the highpass module to help identify dust spots in the sky more easily (and then turn off highpass when you’re done retouching) — this is useful if you’re not using wavelets (as you have other methods when using them)
right-click to remove source-destination pairs on the image (just like masks and other things on the image in darktable)
like most other modules, you can duplicate this one too (from the icon that looks like two overlapping rectangles in the header bar) — duplicating helps you use different modules for different targets on the image (such as one for sky, one for ground), so you can adjust each differently (such as with masking)
And I agree with @WmBrant that you should watch the retouch video from Bruce Williams for everything you’d want to know about the module (including the optional, but powerful, wavelets feature).