Yeah, there is no number to tweak, but every spawned system on the map has a defined star from that file. Changing the star in that file will increase/decrease the spawning of those stars affected. The map generator seems to randomly pick from these systems when creating stars on the map. So the more you have of one kind, the more it should spawn. I haven't been counting but my best guess is that is how it works. (Instead of running through the list over and over when creating star systems) Either way you need to create a list and do the numbers manually sadly.
If you want a completely new file for more accuracy you can create a new file creating your own system group with the values you seem to want. Just change the line <StarSystemTemplateGroup>SystemTemplate</StarSystemTemplateGroup> in Galciv3MapDefs.xml with your new path. There seems to be no limit to how many different star systems you can create in a group.
Anyways, it looks good so far. Hope you get it working 
