I have no idea what the tax forms are like in Mexico. In any case, it would not be the amount withheld, but the actual amount of her tax bill. When you prepare your final tax return, you would have to prepare Mexico first so you can apply the deduction or credit. When you test the scenarios, you can estimate the taxes that will be paid in Mexico but I don;'t know how you will do that; presumably the tax authority in Mexico has some advice or methods for estimating taxes.
