I think you need to play around with the present and future value, and nper formulas a bit in excel (or google sheets).

For example with a 5% after inflation return, spending 5 years in the LCOL location, $20k withdrawals, and a desired final portfolio value of $1,125,000, =PV(0.05,5,-20000,1125000) tells us you would need a total of $794,877.40.

Going the other way, the same 5% return, 5 year time frame, $20k withdrawals and your current portfolio value of $472K, =FV(0.05,5,-20000,472000) tells us you will have $491,892.27

With a 5% return, $20k withdrawals, $472k present value and $1,125,000 furture value =NPER(0.05,-20000,472000, -1125000) tells us it will take 47.33 years.

Don't let the decimals in any of this fool you. There's nothing precise or scientific about this because the average rate of return is totally up in the air and they certainly won't be constant.

In fact, your current portfolio won't even support your desired withdrawal rate using the commonly accepted 4% "rule" let alone a more reasonable (in this CAPE environment. See:

https://earlyretirementnow.com/2016/12/07/the-ultimate-guide-to-safe-withdrawal-rates-part-1-intro/) 3.5% withdrawal rate. So even if you weren't hoping grow your portfolio, retiring with $472k and $20k in expenses would be risky given sequence of returns risk.