If I understand how you modeled it, that calculation appears to have a flaw, in that your cash allocation ignores inflation for ten years. You can't count on getting an inflation adjusted $20k per year for ten years from $200,000 of fixed assets. I ran that scenario in FIRECalc, with every combo of fixed assets that they provide, and success rate was around 50% for ten years.
I'm assuming you figured the $20k/$200k was used the first ten years, then entered $40k/$800K for the next 20 years? What AA? Or did you do something else?
-ERD50
Default AA (75% equities I think). Yep, I ignored inflation for the cash supplied yearly amounts. Though I also ignored any interest earned as well. I figured that was close to a wash, though I have no idea how close the additional successful scenario was with or without the scheme.
I entered $20k withdrawals, $800k portfolio start, 30 years, -20k SS (negative to add an inflated expense) to give $40k withdrawals starting in 2023. I assumed $200k in cash would cover $20k/year (inflated) for the first 10 years. Might need low inflation, higher interest rates, or some bonds to make that happen.