The one-lever fix
line 4(c) = (expected extra tax − already covered) ÷ paychecks left
Most withholding problems — the side gig, the second household job, the bonus-heavy year — get solved by this one division. It converts a scary April number into an invisible per-check trickle, and unlike quarterly estimates, it can't be late.
A worked example
$4,300 of expected side-gig tax (from the set-aside calculator), starting in early July with 13 biweekly checks left: line 4(c) = $331. Payroll takes it automatically, the gig money stays untouched in checking, and April becomes a non-event. In January, redo the math across 26 checks and it drops to ~$165.
When this isn't enough
Very large side incomes can exceed what a paycheck can absorb — if line 4(c) approaches your net check, quarterlies return to the picture. And withholding fixes payment timing, not the bill itself: deductions (mileage, home office) are what shrink the number you're dividing. Both calculators linked above handle their halves.