The calculation is valid so long as Bitcoin appreciates. That won't happen forever; when we're on a bitcoin standard, growth will begin to taper.

I took this idea from the early retirement guys. They favour low-cost index funds, like the Vanguard Lifestrategy Fund. If you assume that annual stock market returns average at 7%, and inflation averages at 3%, then on the whole the funds appreciates by 4% per year in real purchasing power.

So if you skim off 4% this year to cover expenses, your fund is worth as much as it was the year before (adjusted for inflation).

Suppose you have annual expenses of $40k. Our calculation becomes:

$40,000 * 100%/4% = $1,000,000

If you are more bearish on stocks and think the fund will appreciate by only 3% in real terms, then it becomes:

$40,000 * 100%/3% = $1,333,333

Or even worse at 2%:

$40,000 * 100%/2% = $2,000,000

This calculation is all about averages though. Some years the stock market dumps hard, and other years it rips. I believe I've read that the 4% rule has held up even in extreme scenarios like the great depression, because the market eventually bounces back.

This logic should apply to Bitcoin too. While Bitcoin doesn't generate a yield, unlike the dollar it actually appreciates in purchasing power. And so long as it keeps appreciating each year, you can skim something off the top without eating into your overall purchasing power.

The main caveat is that Bitcoin is absurdly volatile. A historic dump in the stock market is a normal day in Bitcoin land. I kinda plucked that 15% figure from thin air, but you could drop it to a more conservative 10% if you wanted to hedge against extreme volatility.