Well, to start off, the reason I picked 50% is the belief that most people, had they not been born, would have been replaced by someone else. Slightly <50% of the world could expect, on average, to be replaced by someone better. (specifically, 0.5*[the proportion that would be replaced])

The reason I chose >50% is that I believe that the population at present is larger than the optimum population. I'm not sure by how much, but I suspect the optimum for our current technology level would be less than 1/3rd of our current population.

Would you like me to explain why I believe the population is above optimum?