If the IRS is being told by a person’s work how much they’re paid, by their bank how much interest they got, by any Etsy-esque services how much they were paid… then the IRS has every bit of information it needs to get automatic withholding correct.

Unless you just want to just keep withholding on the marginal tax rate a person is currently in, thus starting your withholding super low in the start of the year and increasing it throughout the year so that come December they have way more withheld per paycheck than in January, it’s not simple.

Future income is unknown, you could maybe design a system a system that assumes you will adjust your withholding amounts especially in the last quarter of the year to either catch up or slow down with holdings to try to best get you close to zero by the end of the year, but even then to be dead on still is a wild bar to clear as that requires that last source of income to know it’s the last income for the year.