90%. The amount of natural resources the earth provides will only last 200-300 years at most. Our already overpopulated civilization will break down and whatever is left will revert to prehistoric living conditions.
Better to kill 90% now so the remaining 10% can use a second chance not to fuck things up.
The most important thing to remember is that humanity has created nothing....EVER. Zip. Zero. Nada. All we have ever done is to use our intelligence to identify a need or a want, and use the earth's natural resources to achieve the desired We combine, add, subtract and move a few atoms around to make what we desire, but we have never created.

It's a good question, DisPerv. And, although impossible to effect, I have tried to give a thoughtful answer to man's inevitable end, should we continue to follow our present course.