I use my own calculator, not the ones online, then back into what to claim on my W-4. Have you tried that?

Here's how I do mine:

Gross income, minus the standard deduction and personal exemptions, then minus all of my pre-tax deductions (medical, 401-K, student loans etc). This has to be customized to your household of course. You'd have to add in child care expenses and the child tax credit if you get it.

This gives you taxable income. You can figure out how much your taxes will be from this using the tax tables or some quick bracket calculations.

Once you know your total federal tax bill, split it up between your paychecks in whatever way works best for you and use that to figure out what you should claim on each W-4.

So, as an example, say you calculate that you'll owe about $9,000 in federal taxes, you could get $3,000 of that from your wife's checks, which would be $250 a month and $6,000 from you which would be $500 a month. Once you get your withholding just a bit above those (I prefer to get around a $500 refund in case my calculations are a bit off) amounts then you'll be all set.

Does any of that make sense?