I've messed around with MapSizeDefs.xml and .xsd until I'm blue in the face.
<MinPrecursorPlanets>5</MinPrecursorPlanets>
<PrecursorPlanetPercentOfHabitable>0.01</PrecursorPlanetPercentOfHabitable>
It's always 4 or 5 Precursor worlds.
I even tried changing the max habitable planets thinking there was some setting somewhere that based prescursor world on total habitable planets. No luck.
Any of you experts out there figure this out?