Use the cheat mode and generate a galaxy using some middle of the road settings, then reveal the whole map.  You'll be able to see from the unowned planets list just how fruitful it is, and if you zoom out to tactical you can check out how well-distributed the planets are.  Go back to main menu and tweak the settings as needed, then generate another galaxy.

Scattered will result in habitable planets being just about anywhere, while loose clusters means there will be pockets of space with very few stars and very few habitable planets, and tight clusters goes so far that each cluster is like a mini-galaxy, especially on Immense.

When you're happy with the galaxy setup you can then exit and start the game again in regular mode to generate a galaxy using the same settings.