I often, just for yucks, figure how long my money will last with 0 return. That would be the ultimate "ultra-conservative" rate, to use as a benchmark. Of course the problem here is I am also assuming zero inflation, but the point is to see how long the money lasts. I like to keep it simple.
So if I have $1million and I withdraw $100k per year I run out of money in 10 years. (10% per year) But if I have $2mm and withdraw $50k per year that will last me 40 years, which for me at age 61 is almost long enough. :) So in the second case, $50k/2mm = 2.5%