Does anyone here have deep expertise in the mechanics of US federal income tax withholding?

Mrs pj works full time. I am now working part time, variable hours. They are withholding as if I'm the only one working, even though I checked the box that we both work, but as far as I can tell, there's no simple way to tell them either (A) how much we expect to make total for the year, or (B) just withhold 22% and we'll be fine.

The calculator on the IRS web site is crap and does not give the options I need (I started a month ago and will be working through the end of the year).

Estimate your earning, then estimate your tax bill. Set the amount per pay period accordingly.

Since they changed the system a few years ago, I find making qualitative adjustments to W-4 just not worth the effort.

Much easier to just logon to the IRS once a quarter and make an estimated tax payment for however much extra you need to pay.

If you used tax software to file your taxes last year,an easy way to bake an estimate for this year to just take a copy last year’s return file, and plug in your estimated income numbers into TurboTax or taxcut or whatever you used.