I agree--this would be great.

Map spawn issues are also mentioned in this to:

https://forums.galciv3.com/482160/page/1/#3670533 

My reply #6 has my thoughts. I dont mention Precursor Worlds but the same logic with the map generation of extreme planets and 'planet' resources has the same issue. It would be great as you suggest to get more options and control over all. Saves from making maps get stale as well as, as you said, allow for more tailed game play based on your map settings.

There is no randomness to the current precursor worlds, they are a set number, no %, set in stone number. So it doesnt even vary by your ingame planet settings. Same amount, every game for the map size you play.

I think that would be a good idea as well; some sort of scale, button or whatever to alter the setting.

It turns out it doesn't matter. Just like the "proportion of extreme planets" field, it appears that "proportion of precursor worlds" field has no effect, at least not in the map editor (I didn't bother checking the actual game, as they seem to correspond when the setting are the same). So, your original statement turns out to be correct, and the answer to my question is that, yes indeed, that field is non-functional!

As always, talking to you about planet modding ends up with valuable information. Thank you!

Have you ever actually counted to make sure the percentage is correct? The reason I ask is that when I set extreme planets to, say, 99% or 100%, it gives me about 50-50 extreme and non-extreme.

EDIT: FYI, I had set habitable to 1%, and almost every star had a habitable planet!