[Strategia_In_Ultima]

Or, what you could also do, make the first few M pop easier to kill, but as you depopulate the planet you'll need to do more and more damage to kill the same amount of population. The amount of damage you need to do is calculated using some sort of formula involving the size of the planet, the size of the original population, the size of the maximum population, one or two random or semi-random factors and perhaps a few other variables.