You might as well bump up your allowances by a couple right away. No matter what you do you're likely going to have too much withheld this year due to the $2,000 child tax credit that your previous W-4 settings weren't anticipating, so reducing your withholding by a bit right away isn't very risky. If you really want to do some math, you might be able to get away with dramatically increasing your allowances through the end of the year and then resetting them to a more reasonable level in January, but this seems like a lot of work just to avoid loading the IRS a bit of money until you file your 2019 taxes in a few months.
At some point you can use your favorite tax calculator to estimate your total tax liability for 2020 given your wife's partial year of work income. Once she quits you can look at how much you have each had withheld for the year, figure out how much is still left in the year, and divide it by the number of remaining paychecks to see what your withholding should be for the rest of 2020. Compare that to how much was already being withheld to see what sort of adjustments need to be made. This can totally wait until March. Plenty of time to correct your course either way at that point.
