The "live off your bond allocation" or "live off an N-year cash buffer" ideas are alternate ways of basically saying: "increase your stock allocation percentage as the market P/E drops" optionally combined with "start with a more conservative asset allocation".
- These are both strategies that increase your risk profile while the market is tanking. This may be scary to implement. Or you may believe that low P/E times are great investment opportunities and you like the idea of increasing your risk. Either way you look at the scenario, make sure you are comfortable with what you're doing to your asset allocation.
To OP's 20% market drop concern, historically 43% of retirement start dates have seen your portfolio value drop 20% below it's starting line. Those are good enough (bad enough?) odds that we shouldn't be at all surprised when this happens. See Dr Doom's "Oh Shit Percentage" (OSP) idea under the "3. Living Below The Line Is Normal" section here:
https://livingafi.com/2014/06/03/drawdown-part-5-validation/, in particular look at the "Dip Analysis" table.
Personally I found the OSP idea very calming: we've identified items in our family budget that will go should we hit our OSP, so there's a back up plan in place should we need it.
Good luck figuring out your draw down strategy!
--Michael Sheldon
* Why do I say "increase your stock allocation percentage as the market P/E drops"? Because that is in effect what you are doing. Let's look at a couple examples. The math here assumes the 4% WR exactly equals one year of expenses, so this is basically "I just started RE and the market tanks" -- if you've seen market growth so currently use a smaller WR then the increase in stock allocation isn't as much, but it still happens:
3-year cash buffer idea. The cash doesn't contribute to your 4% WR, so you need to save this extra 3-year cash buffer. Which results in a just over 10% allocation to cash and the rest to stocks/bonds. Assuming 70:30 ratio there, you have: cash 10.7%, stocks 62.5%, bonds 26.8%. In a long extended down-turn, you spend your cash buffer and wind up at a 70% stock allocation. You increased your stock allocation.
just-spend-bonds idea. You are in "just sell bonds" mode. After the 20% crash your 70:30 ratio is now 65:35 (simplifying assumption: bonds didn't change value, stocks are flat going forward for a few years). Sell bonds to get your next year's living expenses and you are at 68.3:31.7. After 3 years of this you are at 75.7:24.3. You increased your stock allocation.