As far as I can tell, the only thing PQ determines is how many tiles you get to put projects on, on a planet. Yet I've tested carefully now, and if you use a race without a PQ bonus and colonize, for example, a quality-12 planet, you get 11 base tiles to work with, plus 2 yellow ones requiring Soil Enrichment. Then, if you use a custom race with a +20% PQ bonus and colonize a quality-12 planet, the planet's quality changes to 14--and you get 11 base tiles to work with, plus 2 yellow ones. And it's exactly the same for any quality of planet from 3 to 15.
So, is my base assumption wrong? Does Planet Quality affect something other than the number of tiles you can work? If so, what? What is it about PQ that makes a mere +20% worth 8 points?
Thanks.