I'm pretty sure everything is just simple addition/subtraction. Watching the video, neutral planets don't have any growth. When you get a planet to -1 you take

I'm pretty sure everything is just simple addition/subtraction. Watching the video, neutral planets don't have any growth. When you get a planet to -1 you take control and are left with 0 ships there. (In the video green sent a fleet of 50 to a planet with 4, green took control of the planet and had 45 ships there)