@Moogle65535. That just defines the planets, not how frequent they are.
@emmagine: The frequency of all PW was changed dramatically late last year, early this year. The frequency can be found in the file MapSizeDefs.xml
If you want to modify look for <MinPrecursorPlanets>XXXXX</MinPrecursorPlanets> and <PrecursorPlanetPercentOfHabitable>XXXXX</PrecursorPlanetPercentOfHabitable> under each map size and adjust as one sees fit.
Then save the file in the mod directory under the EXACT same name, enable mods, and enjoy. 
For the record, <MinPrecursorPlanets> sets that there will be at least that number of precursor worlds in a map setting (though one might not be guaranteed to see lvl 2 ones) while <PrecursorPlanetPercentOfHabitable> sets the percentage of all worlds that can be colonized that will turn out to be precursor worlds.
As an example, at the start of 1.5, a Medium sized map was guaranteed to have at least 9 precursor worlds with the PrecursorPlanetPercentOfHabitable variable set to 0.05.
Now under 1.7, a Medium sized map is guaranteed to have at least 3 precursor worlds with the PrecursorPlanetPercentOfHabitable variable set to 0.01.
Bit of a difference. 