Since the angels know my preferences, I'd set it very high.

Curing the world of abject poverty and rampant resource use by noncontributing third worlders would save the planet. We could easily shed like 2-3 billion people.

But the more "humane" way to do things would instead have the dial force those same people to suddenly become sterile. Then you could set the dial to 70% and be assured that global birth rates will plummet and we'll coast for a while before dropping off for the better after a few decades. Countries that need more births will instead be granted what the third world lost.