Looks great, and yeah I think you got it!

I did some more reverse engineering in the meantime. The detail, I think, is directly related to resolution of your image and your cloud. Did you end up using a plasma? I think you have to, but it may be that the others work too.

To adjust the brightness, play around with the levels (color -> levels) to control the brightness of your cloud layer. Drag upper slider to the left, lower slider to the right:

Click image for larger version.  Name: 05-levels.png  Views: 56  Size: 36.7 KB  ID: 65763

Play around with it a bit, it's hard to describe for me but I think you should get the hang of it quickly.

This is what I got, using a section of the milky way galaxy as a base. I didn't draw any custom shapes this time:

Click image for larger version.  Name: reverseengineering.jpg  Views: 137  Size: 844.7 KB  ID: 65764

The levels in the screenshot may not be EXACTLY what I used to get this image, but you get the idea. Oh, for the plasma, I used random seed 42 :-)

Last but not least, I used this little piece from the milky way image and scaled it up:

Click image for larger version.  Name: apieceofthegalaxy-small.png  Views: 96  Size: 160.5 KB  ID: 65765

As for the star layers:

It's not that I use it for spectral classes or anything so scientific, just to keep stars separated so that if I decide to move something, I don't have to re-do everything else in its neighbourhood as well. If that makes sense? I placed a lot (well, not all) of them fairly deliberately, either to match my campaign setting, or to highlight "boring" parts of the base image.

Note that when I say "stars" here I mean the ones that are part of the background graphic. The colony worlds (white circles with black outline) and their labels etc were all added in Inkscape, not gimp.

The star brushes were cutouts from NASA star field photos. It seems I lost them when I re-installed my PC.