@thompsonmark Haha I'm a tax attorney and struggled with it until I ended up just reverse engineering it based on the amount of taxes I know I'll owe.
You take the standard deduction for your filing status, divide by the number of pay periods, and exclude that much from withholding. Then, you add back the amount earned at any other job, as that job will also assume you get the benefit of the standard deduction. The subtract from the withheld amount the portion of the CTC, ACTC, EIC which would be attributable to that pay period.
I've seen some real disasters where both parents claim the kid, or the phaseout isn't correctly figured for the CTC (less relevant in this sub). Whenever I do a tax return, I tell the client what their W-4 should look like to get no refund the next year, but this year that's a little more work than usual.