Paying Estimated Taxes

This gives guidance:

Basically, if your joint tax was X, you figure what husband and wife would have paid if they had filed separate tax returns last year, and then compute (X * H) / (H + W). That's your safe-harbor tax for prior year.

They give an example:

"Joe and Heather filed a joint return for 2019 showing taxable income of $48,500 and a tax of $5,435. Of the $48,500 taxable income, $40,100 was Joe's and the rest was Heather's. For 2020, they plan to file married filing separately. Joe figures his share of the tax on the 2019 joint return as follows. Tax on $40,100 based on separate return $4,686 Tax on $8,400 based on separate return 843 Total $5,529 Joe's percentage of total ($4,686 ÷ $5,529) 85% Joe's share of tax on joint return ($5,435 × 85% (0.85))"

But you do know that he doesn't necessarily need to file separately this year? A nonresident alien married to a US citizen can still opt to file a joint return. (Details in Pub. 519.) Of course there might be some reason why the wife would not be eligible, or be eligible but not amenable to that.

Perfect Stan. Thanks.

I hope you know that there are facts missing from your post. Surrendering your "green card", i.e., giving up your lawful permanent residency does not necessarily mean you are no longer a US resident for tax purposes. You state, that THEY are in CA. Sounds to me that she is still a US resident under the substantial presence test. As such, they can file a joint return or they can file married separate returns. As they both are in CA, they would still have community income unless they have taken overt acts to break the community.

Yes, I realize there are issues that were not addressed. I just took the husband's statement at face value - in this instance it wasn't my job to look beyond that.