It's the government we're talking about, so they aren't overthinking anything...because they know better than us, they are saving everybody from themselves by making it as simple as possible for the least common denominator, thus withholding levels that match the income tax brackets (plus a 7% thrown in for those whose income is low enough that they are below 10% after the std deduction). And, the government doesn't mind if their break points result in you giving them more money to hold until next year.
I take my inherited IRA RMD in 2 quarterly payments, and cover my safe harbor requirement by withholding with those 2 payments (I can choose the %, and I can take a larger amount than my RMD requires if I want/need to). Now I'm done withholding for the year. The 1st quarter, I technically could withhold $0, because it is less than my standard deduction (i.e. 0% tax rate), but I know pretty well what my interest and divs will add up to over the next 12 months, so I withhold for that now. The 3rd and 4th quarters I'm using dividend and interest from non-retirement accounts. I can take extra from the inherited IRA in December if I want, and can have up to all of it being withheld, or I can sell something from my brokerage (tax gain harvest?). I'm one of the low spenders, so I don't pay much tax anyway, and don't have need to do Roth conversions with my modest IRAs (I have a big brokerage acct, though
).
I also keep a tally of my withdrawals/distributions for the year, and separate them by income or ltcg, and know where the I am relative to the tax bracket breaks (taking into account the standard deduction), so I know what tax rate my next withdrawal/distribution will have. One could draw the little bar graph, that's often used to demonstrate filling the vertical levels of income tax brackets and next to it the LTCG brackets, and could sketch in where they currently fall as to what tax rate your next distribution will require to be withheld.