I learned a lesson last night. I was playing a game where everyone just got their home system. I had a 300% farming bonus on my home planet and put a farm on it. Near the end of the game I suddenly started running a deficit that I couldn't get rid of. I tried a couple of things to no avail and finally tried decommisioning my farm. After the population went back to 10B on the next turn I reset my tax rate and everything was ok. How do you tell if you've got more than the optimal population?