For example, in this image I have 10% below zero volcanism. This makes sense to me, 10% of the map will basically not be fit to host a volcano. 50% of the map is below the igneous extrusive threshold. 70% of the map is below the volcano threshold, if i'm reading this correctly, the remaining 30% of the map is eligible for volcano placement.
Sorry for the late replies you two! Volcanos only appear at full vulcanism which is above the top slider. Also you must set the number of volcanoes in the map parameters.
Imported a world gen with elevation map from an earlier region. On the elevation page, the sea and tree levels didn't adjust to the ones in world gen, and I couldn't just set the numbers so I had to tune the %s by hand.
The default smooth brush is still a little too defined to look natural, but I couldn't undo or erase. How about a feather radius setting? Smear and blur brushes? An option to permit noise to distort brush strokes could also help blend it in.
EDIT: The manual says exporting world_gen.txt would be enough to build according to parameters generated with PW. I didn't believe that would do that, and I was right. I confirmed the worldgen data in my df installation was updated, but the world generated wasn't any different from one I would have generated with the data I imported into PW.
You should be able to undo the brush with ctrl-z. If I remember correctly I went overboard on this function and you might actually be able to undo it even after saving it as a pwset file.
The way the import export works is like so: If you import a world_gen.txt file that already has data in it, it will preserve that data unless you choose to throw it away. It doesn't actually import that data into PWDF for further editing. If you want to save a world created with PWDF, you have to save it as a pwset file if you want to edit it again. You can't restore your PWDF info from the world_gen.txt file.
So in summary, there are two files here. There's the world_gen.txt file that is there purely to interface with DF, but saving your work has to be done through the pwset file, which you save in the 'File' menu under 'Save Map Set'.