I don’t think there’s much need for heuristics like “rate of effectiveness change times donation must be much smaller—say, a few percent of—effectiveness itself.”
If you’re really using a Landsburg-style calculation to decide where to donate to, you’ve already estimated the effectiveness of the second-most effective charity, so you can just say that effectiveness drop must be no greater than the corresponding difference.
I don’t think there’s much need for heuristics like “rate of effectiveness change times donation must be much smaller—say, a few percent of—effectiveness itself.”
If you’re really using a Landsburg-style calculation to decide where to donate to, you’ve already estimated the effectiveness of the second-most effective charity, so you can just say that effectiveness drop must be no greater than the corresponding difference.
That’s an excellent point that I managed to completely miss. Thank you. I’ll try to add an endnote to that effect.