As a fellow engineer, I appreciate your intense detailed approach
.
I would personally say that you can't use a hard-and-fast rule on your question: what if one of your holdings distributes a big taxable distribution in your taxable account one year? Or if you sell a position due to rebalancing or a large run-up? You sure wouldn't want to also have made a large withdrawal from your retirement account earlier in the year, since that would jack up your taxable income even more.
I would say approach retirement with 1 year (or more) of expenses in a MM account. Using that as your spendable cash, as the year goes on, you will see what your taxable account brings as far as taxable sales/distributions/dividends. Then, as the end of the year approaches, you will know exactly how much you 'should' be taking out of retirement accounts to minimize your taxes - or if you should let it ride and sell taxable positions with capital gains (if cap gains rates are still this low in the future).
There are also points to be taken with delaying SS until age 70, and taking out of retirement accounts before age 70. This has a two-fold effect:
1) Delaying SS increases your SS benefit permanently (only worth doing if you have a reasonably good health history in at least one spouse's family).
2) Drawing down your retirement accounts up to age 70 leaves a lower balance to take Required Minimum Distributions from when you turn 70.5 (which is also the age you could take SS at
). If you never take out of your retirement account until age 70, your balance will be much larger, and your subsequent RMD will be that much larger, possibly throwing you into higher tax brackets. If you start taking from your retirement account earlier, your RMD will be lower, and thus help reduce your marginal tax bracket.
Of course, if you SEPP your accounts, your strategy will be modified for the 59.5/5 year duration.
To sum it up - depending on actual retirement age, you could SEPP it until 59.5/5 years, make up the difference (if any) with taxable account sales. Then, at 60, review your account balances, figure out income needs, and calculate your withdrawal strategy for delaying SS until age 70, with an eye on taking as much out of your retirement accounts (to reduce age 70 RMD), while still not ballooning into 2 higher marginal brackets because of large retirement account withdrawals.