An immortal, such as a corporation, has to use a safer number, such as 3%, or 2.5% for operations and 0.5% in fees for the fiduciary management. So the permanent endowment needs to be 40x annual operating costs, and the fiduciary needs to grow it by 3% better than (price) inflation per year. That's relatively easy to do when most of the principal won't be touched within the next 30 years: buy all the publicly-traded stocks that have historically paid regular dividends, and reinvest whatever isn't paid out. On a long enough time scale, that's probably 7% better than inflation.
So the fiduciary could possibly be replaced by a robot that only needs 0.05% annually for maintenance, and then you'd only need to endow 34x annual operating costs to run forever.