In this case the more important thing is to place the samples where there are no stars. If you see your previously posted image, it has dark areas. That is because there were samples covering stars or bright parts of the nebula.
It is useful that you apply a simple DBE first on an auxiliary image to have a better visualization of where you place the samples. Once all the samples have been placed,
generate the icon and reopen it on the original image.
To generate the samples, you can both, apply them one by one, or Generate a considerable amount of them setting the number in the "samples per row" box in the Sample Generation section, and then check one by one to move them away from stars or bright nebula areas.
In this case I only have increased the Tolerance parameters. That was to be allowed to place samples in the bright parts of the image (if you do not increase it, you will see that the sample is red, which is a bad sample)
You can see also
this video regarding DBE.
hope this helps
