Each population point is worth its number * 10,000, but the effects are cumulative, ie pop point 1 = 10,000, pop point 2 = 20,000 PLUS pop point 1, etc. And the additional population is calculated with the amount of food you have towards growth, so pop 2 with 9/20 food has 10,000 + 20,000 + 30,000 * 9/20 = 43,500 population.